Our store sells it for $160 for 3 gallon kit, we are in FL. There are more expensive brands but for what your doing the less expensive general purpose is more than adequate.
http://boatbuildercentral.com/products.php?cat=41
Good, fresh, iso-polyester is going to be more than $20/gallon. If you see "boat yard resin" it is old leftover polyester, literally bottom of the barrel. Polyester has a shelf life too, its promoted before its packaged and is slowing curing unti l you add the catalyst (then it really cures).