Well let's just say over the last few months the count has grown to about 150. (I have a train hoarding problem)...

Of the 150 probably 50 are childhood junkers and father-in-law projects that are not worthy of the investment.
I plan to be able to use straight DC when using the old stuff. Also plan on unloading quite a few and using the cash to buy RP stuf and other train stuff ... track/switches/cork RB/ .... more loco's

I would estimate that overtime - I may Railpro around 50 locos. Being HO, a LM3s, speaker, and KA, maybe a 8 to 9 pin harness ...
Lets say about $150/ea x 50 = $7500, even if the number drifts north of that ... say $10K (66@150) ... still not terrible, especially over time.
Also, probably won't use Sound and KAs in all of them, so a good portion will have no sound LM3s which are about $70 ...
Compared to what some sink into their dcc setups, the Railpro system is certainly more economical.
G scale is certainly in a class of it's own tho, agreed.