I've heard that you can order PG from the Wal-Mart pharmacy for $20 a gallon.
I get mine from a local tractor supply store for $20.99 a gallon.
The best price I've seen online is Amazon that sells a gallon for $20.99 as well, but since it's from a vendor, you have to pay shipping which puts it up into the realm of $30 a gallon. If Amazon themselves stocked it, I'd be on that like White on Rice on a paper plate in a snow storm. But sadly, they don't.
My suggestion for the PG would be to check your local tractor supply stores.