New barn floor

I'm building a new hay storage barn in a generally wet climate. Can't use rock or gravel as the plan is to unload a pull type bale wagon. Regardless of the thickness and compaction I'm pretty sure the bale wagon forks would drag the gravel. Was thinking about concrete with a plastic vapor barrier underneath but concrete is one - third the cost of the building itself. Is asphalt a bettering more economical choice and if so should it have a vapor barrier underneath.
Any thoughts would be appreciated.

New barn floor