Fridge Logic: Shane's fall from grace is ostensibly due to the difficulty of maintaining a family on $39,000 a year. And yet Nathan can apparently take his daughter to a fancy restaurant and buy her a diamond-studded promise ring on the same salary... admittedly, Nathan may be at a higher pay grade, or his wife may work, but his daughter specifically points out that normally, they wouldn't be able to afford going to the restaurant.
Nathan could have been saving up his money for that special occasion. Also, Shane could have been wasting his money on things that drained his budget more than he thought.
