But that's an insane quantity ! They're microballoons - the whole point is they're ultralight.
My tub of phenolic balloons (West System) cost me a "few bucks", I use them for almost all my filled epoxy, and I'm nowhere near using the tub up yet. For a boat with faired mouldings then maybe they're expensive, but for thickening glue then I spent more on shoeleather going to the shop then I did on buying them.