Hard to say these days, the market is so bad and so in flux. I think you should start by figuring out what the motors are worth and then add $10-$15k for a first offer. That's harsh for the seller and I'm sure some here will think it unfair, but I bought a 2002 2520 last September for $30k with a brand new F225 on a bracket (approx $20k) and a trailer, and that's 10 years newer. Now I still think that was a screaming deal and these boats don't deteriorate much so the 10 year difference doesn't mean as much as it might with another brand. On the other hand, the other boat I was looking at at the time was an early 1990s 2520DV with twin 150s (also bracketed) and its owner had already done a lot of work to replace some soft decks and the fuel tanks. I think there was a point in the early 1990s after which they started glassing the underside of the decks and it took care of some of those problems, but the point is that it's not like you can't encounter problems with that vintage. I think he wanted around $40 for that boat. The surveyor I hired said the work was solid and it was a good deal and I would have bought it if I hadn't found the other one. So get a good survey, and that's a decent data point for your value -- $40k if the necessary work has already been done. If the one you're looking at has a nice pale yellow paint job you should contact me because it might be the same boat and I may be able to dig up the survey report I paid for.
For what it's worth I have LOVED my boat. It's still in the water now, and floated on through last month's 25 year record snowfall on the Chesapeake without a hitch. (Oops--too late to do anything by the time the weather turned awful). Every boat is a compromise but a 2520 is a damn good best of many worlds. Superb fishing boat, passable family speedboat, and pretty serviceable weekend cruiser when you want to explore and camp out--all with good speed and fuel economy.
