The real saving comes in when you shoot colour trannie and it is huge!!!!!!!!It is the processing costs that kills you.
5x4 Velvia quickload - £2.50 plus processing £3.50 = £6 So for each image, I conservatively assume you shoot 2 sheets (OK sometimes your exposure will be perfect, but you will still process that second sheet for a spare or for a teensy weensy tweak. Equally often you will very much need that second sheet for push pull. Therefore your cost per image is approx £12 minimum assuming you do not actually shoot 3 sheets....exposed differently, processing the best guess first and then doing the others....
A roll of 120 is £2 mail order and the same at £3.50 to process = £5.50, so half the price for 2 different bracketed images (3 frames per image, 6 frames per roll)
The cost for 6x12 120 is therefore £2.25 per shot and £12 per shot on 5x4 quickload. Thats £10 per shot saving, again assuming that you do not even bracket on sheet, but do a second exposure only.
£10 x 30 images =£300 ie the cost a second hand Horseman 6x12 back. Plenty of people would shoot that in a 2 week photo holiday, or for a pro doing holiday resort shots or architecture/interiors (for example), a few days....
I agree tho that for mono, its crazy, which is why I dont hve one. also, it is yet more weight to carry.