I mean, the card is from the 500 series, which some of the cards are still good. But for that one, games today may struggle at higher settings. That thing is sure as hell better than my low profile 5570, lol.
If I were to guess, and assuming that it can run games like BF3 at medium settings @ 60 FPS, it would probably be anywhere from $30 to $150, dependent on how worn it is, how hot it gets, and how good it plays games.
EDIT: If you have any benchmarks or just know on average how many frames it gets at certain settings on certain games, please provide them.
EDIT (2): I see the GTX 550 ti on amazon used going for ~$90. If I were to guess, usually non-Ti is cheaper, so I'd say, no less than $50, and no more than $80 to sell it for.