I wouldn't say they cost more than the gun is "worth". Definitely more than you paid for it though.
Bolts tend to be the part lost most often on these. Since they're no longer in production (company was bought by KEE, and their entry level stacked tube blowbacks are based on the PMI Piranha) replacement parts are drying up pretty fast. You might be able to find another gun for the same price that's nearly complete, just make sure that one has a bolt. Parts should be cross compatible with most Viewloader blowbacks and later Brass Eagle metal blowbacks (like the Afterburner, older Avengers are a bit too different).
A bolt from a Stryker STR-1 might fit...
...but no guaranties.