Yup, you got ripped off. I got mine at Home Depot, 3/4", three sheets, and it was $90 and change (which includes the tax). I should be $25-$30 a sheet, definitely not $40. It's just sawdust and resin, no excuse for those prices. Getting the $30 might not be worth the effort of hauling it all back (that 'ish' is heavy!), but see if they can price match, based on your receipt. They might actually. If they don't, assuming they don't have a policy against it, threaten to bring it all back if they don't price match.