Metal is cheaper to get size for an outbuilding. If you're going to "finish/insulate" the inside, you'll likely frame it out in wood anyway. Only other option would be to have the inside spray foamed.
Like Adam said, 24x30 would make sense in wood. Amish are building me a 28x30 for $16k, everything but the siding (I'm doing concrete siding), and the concrete floor. They'll have it done in 2 days.