Here's a very rough estimate as size can vary from machine to machine and even from bale to bale...
small square bale = 1.5'x1.5'x3' = 6.75 cu ft
large square bale (your dims) = 4x4x8 = 128 cu ft
So about 19 small bales in one of your large bales. Prices vary by region and by type of hay. Here they can be anywhere from 2.50 - 4.00$ each. At 4$, your large bale would be worth 76$.
Sorry for the fluffy info.
Paul