If Gerst built a coaster similar to a B&M one, it would still be really expensive, but probably not as expensive as if B&M did it. Anyway, Gerstlauer doesn't tend to build big rides, because even if they would be cheaper, they probably couldn't compete with B&M, Intamin and the other bigger manufacturers.
I also believe that (keeping our examples) B&M and Gerstlauer are not direct competitors, since B&M wouldn't want to build a ride for Gerstlauer's prices and they probably wouldn't want to build such small rides for image reasons, and Gerstlauer won't get a contract for a B&M sized coaster, because parks would want to put such a big investment into the hands of someone who has proven themselves many times.