Hehe, we did this conversion once, using the price of a full grown cow as our measuring stick. Unfortunately that was in 2nd edition and now there's no price for cows that I see in the SRD.
So, let's go for meals... the price of meals stays about the same compared to everything else, right? So... a poor meal would be like a cheap McDonald's lunch, call it like $4. A common meal would be at like TGI Friday's at about $12 And a good meal would be a steak at the Outback at about $20. This all corresponds to 1 sp, 3sp, and 5 sp for those meals in the PHB.
So, if we assume 1sp = $4 (US$) then we see that a gold piece is about $40. Which makes a night's stay at a good inn $80, which seems reasonable. A mug of ale is $1.60, which is reasonable considering the quality of most of the ale back then.
Some of the prices seem to be kinda far off using $4 per silver piece, but it's not too far off.
-The Souljourner