You're comparing two different products - West Systems is a 100% solids epoxy resin while Sweetwater is an epoxy paint with solvents mixed in (so less solids per volume) so it doesn't make sense to compare cost on a per volume basis. That being said, WS is one of the most expensive marine epoxies and there are several cheaper options for resins available - US Composites and Max ACR are two that come to mind.