Final Seal is NOT that expensive in the 2oz bottle unless you're seeing it being sold in a shady costume shop. SillyFarm has it available in that size
for $7USD, but I'd just fork out $18 for the larger 80z bottle, as you get 6 extra ounces for close to double the price and it will LAST you.
But if you still want to go cheaper, you could give this
a try. I've never used it as I have a giant jug of Final Seal, but at $9 it couldn't hurt. Just look around for some reviews.