A $50 bill will convince many stone shops to run they on their saw. All 4 sides can be squared in about 30 minutes. The edge will not be polished, but will be square.
There are now a lot of bottomfeeders in the stone countertop industry. I looked at a Chinese import Granite slab yesterday, 8' long, polished on the top and two sides for $79. The slab you have is thicker (much better) but it is hard to beat the prices that are showing up.
Aaron