Normally, I'd give the standard "check completed listings on ebay" answer. But since I'm a little curious myself, I'll do all the legwork for you. I'm including the shipping costs with the total prices, BTW.
" Atary"7800 complete- with poor description and no pictures- $32
7800 w/ hookups and 9 games $41
7800- working with 37 games $51
Complete Boxed 7800 with 12 games for $116.50
Anyway, this is just a sample, as there are quite a few completed auctions. If you don't have an ebay account, I'd recommend signing up for one, because it's a good indicator of retail value for a lot of things. ("Retail," being the maximum value for an item.) You also have to take in consideration, the games that are included, because some might be rarer than others, and inflate the value of the system. Rarity is also meaningless to a a non collector, who just may want the system for nostalgia's sake. Also controllers or other parts in poor shape will also lower the value.