Plus, if a Shotgun cost more you couldn't have a bunch of Shotgun wielding Cultists running around! 
Oh yes you can !

In SA currently, for 15 BP you can have a Cult Leader (5BP) and 5 Cultists with Shotguns (5 x 1+1 BP)
With our revised cost you can have a Cult Leader (5BP) and 4 Cultists with Shotguns (4 x 1+1.5 BP)
I don't find it hampers my evil creativity too much

but at the end of the day it is only a House Rule.
I do have a copy of Shocking Tales #1, which I think is great, however I feel the ammunition types actually magnify the problem, you in effect end up paying 2BP for the .45 (1BP for the .45, 1BP for the ammunition) when you could buy a Shotgun for only 1BP. Whilst for Threshold models this is not too big a problem, for Lurkers I would go with the Shotgun every time.
I should say I have no problem with any of the other weapon costs and find them all well balanced, it is just the Shotgun that I feel is too cheap.