I think concrete was roughly $120 per yard the last time I needed some. Gravel about $25.00 per yard.
Concrete requires framing, rebar, raking, floating, trowelling, etc....(beer and pizza for the guys I would need to help)
Gravel can be raked over a weed barrier. (wife/son helped--they already eat/drink here).
I would love to have a pad to roll around on, but for less than the cost of a single yard of concrete....I placed, levelled, 2x2 concrete patio stones for each tire and the tongue, filled in the rest of the space with gravel.