Also, where are you located as it makes a BIG difference (basic economics re: supply and demand)? I've seen stuff in New England sell for thousands more than the market would bear here in FL; granted the northern boats are usually not destroyed by UV like ours are.
In my opinion,
if the moter is 100% (compression, carbs, costmetics, maintenance, etc.) and the trailer doesn't need all its springs, tires, bearings, wiring, hardware etc. replaced, then it's probably worth the asking price. You'd basically be getting a free boat (albeit one that needs some serious work). Now, if the engine is "just another mid 90's 225" (i.e. rusted leaking tilt/trim, cracked faded cowling, rusted steering arm, runs rough, etc), and the trailer needs new tires and springs or more, then the seller is definitely aiming a bit high.
Basically, it sounds like it's priced in the right neighborhood, but sight unseen I doubt it's a steal.