The plastic walled ones will be cheapest, and more easily ventilated, but the aluminum ones will be more easily come by.
Ironically, if memory serves,the steel walled ones will be cheaper than aluminum.
But they'll also be the hottest, most humid spot on earth besides the rain forest or the inside Rosie O'Donnell's skull cavity.
I also think region you're looking in plays a part in price.
Don't have any ideas at the moment as to cost, but last I knew a good sized shipping cotainer was somewhere in the ballpark of 8k.
for $8k I can build a workshop as big as my house.