The answer is "not much". Usually it's the peripheral gear (computer, monitors etc) that have secondary value so the card/software on its own, especially Vincent 601 / 7.5 combos, is usually very cheap. They regularly come up on e-bay, or I'm sure that there are many people on this list that have spare systems that they'd be happy to pass on to a fellow user.
It is worth noting that Media 100/Boris are still offering the "Tune Up" deal that, for a fixed fee, upgrades any version or software level to XR and either 7.5 or 8.2.3a. This means that you can buy a really old/low board for generally not much at all and then upgrade. I recently saw a Media 100 QX v 5.0 go for GBP 9.99 on ebay, and I myself bought a p6000 for GBP 10, so it is worth considering as an option.