Jonathan, if you would shoot 100,000 frames in the next 18 months instead of 50,000, you could save even more money on film/processing. Maybe even become wealthy.
Shooting a lot of frames does not necessarily equal saving money. There needs to be an increase in productivity. Before you ask, my point is: Would you have shot 50,000 frames of 645 in the last 18 months?
50,000 frames in 18 months is more than 90 frames a day, each and every day, rain, shine, holiday, sick day, day off, etc. Can anyone shoot 90 MEANINGFUL frames a day for a year and half? Take some time out to eat, sleep, urinate, deficate and read the LL, you get to about 8 an hour. Take some time to edit those frames and the time per shot is really small. I expect your trash rate must be (or should be) very high.