There’s what the game deems it to be worth, and what it’s worth to other players. Sometimes two very different numbers. The first is the easiest to find out – go to Kyoubai and click “create an auction” and the ‘base cost’ of every auctionable item (it has to be in 100% good condition) you have will be listed next to the items. You can sell anything to the ppwn shoppe for some fraction of that base price, depending on your Frugality stat (see the wiki article on Frugality for details).

The more interesting (aka normally higher) figure is what a thing sells for at auction, which depends mostly on the stats of the item – anything with +7s will sell well, especially if they’re in Offence and Defence (for pwning) or Luck and Charisma (for good deliveries), everything else declines in value because it’s only useful to lower-level (poorer) people. Except the really ****ty files… then you’ll be lucky if it sells at all. At some point it’s so common and crappy that you’ll pick up the pieces yourself without even trying.

To get an idea, look at running auctions for the same item – the closer to the end of the auction, the more representative of what you can get the bids will be.