A good running gear brings $250 to $400 without a deck here. It needs to be in decent shape though. Check slack in tongue and steering, wheel bearings, ect. A wagon w/ deck will bring $500 and up. I saw 2 sell at an auction a couple months back for over $1000. (10t EZ Trail w/oak 14'X8' decks and good floatation tires) Some of the really old running gear were a little bit on the light duty side. I like 8 to 10 ton gear myself.
I like 2x6 or 2x8 rough sawn oak deck, 3X12 runners, put together w/ ring shank "pole barn nails". I've never seen a plywood wagon deck that'll hold up to the elements.