Bike value is interesting. My first thought is in the $200 range but then I think would I rather have this bike for $200 or a new bike. My assumption is this is probably a better bike than a new $200 mongoose. You could get something new for the $550 so the value is somewhere in the middle.
A few years ago I sold a clean mid 90's trek 800 I think I sold it for $120 I probably paid $200 for it new. I listed it on craigslist for $150. Knowing that I would see that model and similar models listed for 85-200. I got quite a few $75 offers and it took about a week to sell. The buyer came and looked at it and was happy paying 120 his thought process was that a similar bike store bike would be $300 and it was nicer than any new $120 bike.