Dunno about the capacitor costs But if you were to use even some NIMH cheapies (hack togeather two $10 packs to get 14V), it wouldn't be enough to start the mobile, but propabley enough to smooth out the voltage and keep the spark flowing at low RPM's. Cheap substitute till you get a real lead acid in there.
If I could only draw what I see in my head, then afford to build it, and finaly get to play with it...