Depends on the shape. Check the bunks for rotting, torn carpet, broken bolts. Check the springs for sagging. Check the tires for wear. Check the bearings, hub and spindles for wear. Check the condition of the coupler and the wiring. And just because it is galvanized, doesn't mean it can't rust. I've seen plenty of abused galvanized trailers with rusted weld joints all over the place, and rust spots anywhere it got scratched and wasn't properly taken care of. I would say a "perfect" trailer, 15 years old, with all good equipment, for a boat that is 12 to 14 foot.... maybe $300? I have seen boat trailer places make Jon Boat trailers (galvanized) - brand new, with glavanized rims and the smaller tires, for less than $700. Those prices were seen two years ago in Milton, Florida at a boat trailer manufacturer. Other places may vary. I probally wouldn't go over $300 used.
Glenn