First of all, a new M7 is $4400, so more like 2/3 the price of an M9. Then, at roughly $10 per every 36 exposures for film and developing (no printing) if you shoot just 5 rolls a month, in a bit over 4 years you would be even with the M9 money-wise. And the M7 would then be worth $2000. That's $2400 in depreciation and $2600 in film/processing, or a total cost of $4800. In four years if the M9 is only worth $2200 it would've cost you exactly the same, even before considering all the time you took scanning.
OTOH, buy a used M8 now for $2400 and in 4 years if it's only worth $400, your cost was $2000. Much cheaper than buying a new M7 or a new M9. Cheaper even than if you'd bought a used M7 to begin with.
Staying one generation behind in digital is really the most economical route these days.