My local sign company, yes there's only one because it is a small town quoted me $150 for one sheet. A little more than I had hoped but I'm probably still gonna go for it.My Local Home Depot has some, but maybe its left over as you say.
I would expect to pay about $100 a sheet for a 1/2" 4 X 8 sheet. I think the 5 X 10 sheets were $150 or so.