The problem here is that, at the time you are using as a reference, there was no standard currency conversion in effect for Europe. Each locality issued it's own currency, and conversion was difficult and sometimes impossible. The closest you can come is simply the weight of the precious metal in question. An ounce of gold was an ounce of gold and any two gold coins could be exchanged that way—but bear in mind that the purchasing power of an ounce of gold varied significantly across the continent.
From a more practical point of view—the Venetian ducat was sort of the standard gold coin of that era—I would imagine that someone with a bag of them would be able to spend them just about anywhere.
Madman
I may be getting these numbers wrong, but I think an ounce of gold was equivelant to 3K Us dollars.
That's not far of from current gold prices (about 1.5k) but again we have no baseline.
Research has just made the issue more confusing. For one, the medieval british didn't even use gold coinage until 1344 and even then it was worth only a third of the silver Pound Stirling. Well, there was the gold penny but nobody used the gold penny.
The french used the gold Livre (yes, the same word for book) but the weight and value was inconsistent. In theory, it was a pound of gold. In fact, who knows. Watering down currency with lesser metals was pretty common until paper money.
As for buying power
https://thehistoryofengland.co.uk/resource/medieval-prices-and-wages/
The biggest issue is that the industrial revolution changed all the prices by making complicated goods affordable. I mean, a full day's work wouldn't get you a dozen eggs.
The thing to remember is that, until the industrial revolution, most people exchanged goods and services with neighbors on the basis of trust. They had no driving economic motivation to standardize a currency, or even "prices" (if you have no standard currency in use, then standardized prices have no meaning). If I labor in the fields for a day, my family and I can get enough to eat for two days. What price in gold do you set on that?
Lost in Space
In fact, it wasn't until industrialization that the study of economics at the political or intellectual level even really started.
"It's Occam's Shuriken! If the answer is elusive, never rule out ninjas!"
I think many of you are underestimating the value of a currency system. Indeed, the invention of money predates written records. Similarly, the pound has been in use as a currency sincethe 8th century and was called a "pound" because its value was set to a pound of silver. Similarly, paper money has been in use in China since 118 BC!
Having a working currency system is hugely important for a society to function efficiently as it simplifies transactions and trading of services and/or goods rather than relying solely on the barter system. Also, don't forget international trade.
Sure, the value of gold may vary significantly across a large geographic region but that's true even today. So, just because the buying power of gold differs from place to place does not mean that there was no concept of currency systems or attempts to standardise them.
Lost in Space
That's a startlingly irrelevant post considering that the OP wasn't asking whether there was a stable system of currency, nor did we bring it up.
The question is whether it is possible to meaningfully compare the value of gold in medieval times to the value of gold today, and the answer is that it is not.
"It's Occam's Shuriken! If the answer is elusive, never rule out ninjas!"
I brought it up, but only for the specific time and place that the OP mentioned. We weren't talking about Imperial China.
Night Clerk of the Apacalypse.
Most settings simply make up their value.
One tip I have seen used is to find a rough comparative item for purchasing power to base the currency around. Things like a beer, loaf of bread, a common tool, etc. and decide on a value from there. Something akin to the Koku in Japan is a good example. The rough value of 1 Koku was enough rice to feed a person for a year. Not perfectly accurate but it gives you a rough starting point.
Edited by TuefelHundenIV on Aug 7th 2019 at 5:59:51 AM
Who watches the watchmen?
@Fighteer: My post was in response to your assertion that the study of economics on an intellectual scale didn't start until the industrial revolution, and DeMarquis's assertion that most people exchanged goods with others locally and on the basis of trust.
I was simply highlighting that the OP's question wasn't completely unanswerable as those claims make it out to be. After all, international trade has been a major industry since ancient times and that can only occur if there is a degree of consensus around the value of various currencies.
Similarly, I highlighted how 1 pound (£1) used to refer to the value of 1 pound of silver, again thereby highlighting a frame of reference for modern day comparison. Indeed, many old currencies were based around a reference point that can still be compared with in modern times.
No one is disagreeing with what you are saying in general terms. But the original question was "What was the price per kilogram of gold, silver & copper for year 1500 when valued in todays currency?" in Europe. And the answer was "There was no single price in that place at that time" for reasons that we gave. No one disputes that it might be possible to calculate the purchasing power of an ounce of gold or silver during, say, the height of the Roman Empire, precisely because the high level of commerce across the Mediterranean allowed prices to stabilize. This was not true in feudal Europe.
That's fair. Sorry if I came across as confrontational. It was not my intent.
S'ok. Kept me awake. 
One thing or rather several to consider about ancient currencies is many of them have no reference point with relation to modern currencies because any long-lasting currency (for example pound sterling) surviving since then has often been re-valued, re-calculated, and re-scaled with modern times. Several of them did become Funny Money after all. Additionally, many of the currencies they could have converted to and thus give a reference point themselves were subject to the same thing. Or worst of all in all this, the currencies involved no longer exist owing to political shifts, conquests, wars, economic changes, regime changes, new systems of government, new systems of economics and more. After all, nobody knows how much of anything a single Spanish gold doubloon from the 1500s-1600s bought despite the Kingdom of Spain (Reino de España) from back then still existing today.
There's also one more thing. Prior to the Age of Enlightenment (1700s on later), much of economics and trade also revolved around barter and price haggling so there was very limited if any price stability or relations of prices of one thing to other goods and services.
Edited by MajorTom on Aug 10th 2019 at 6:05:10 AM
I'm making a story set in the late middle ages and I need some help with the currencies. The setting vaugely resembles the last quarter of the 1400s and I want to make the currencies & their values that might show up at least somewhat realistic for the period: What was the price per kilogram of gold, silver & copper for year 1500 when valued in todays currency? (You can choose say £ or $.)
I reject your reality and substitute my own!!!