If you develop yourself, you can do a roll for something like 10 cents' worth of chemicals. Obviously it requires a bit of investment up front, but not much ($50-100 for some graduated cylinders and funnels and bottles and a tank and blah blah). Once you get good at it you can even do many rolls at once, so it becomes not a big investment of time, either.
Obsidian Aqua -- cheapest developer I know of, about 3 cents a roll
Most stop baths -- about a penny a roll
Cheap fixer re-used as much as possible -- a few cents a roll.
Wetting agent and/or distilled water - negligible, pretty much. Maybe a cent a roll.
The trick is then to not just order prints for all of them, but only the ones that are actually good! Do you order prints for all your digital photos? No. Just like keeping most of those in a folder on your computer and them never seeing the light of day, it's okay to leave most of your negatives in a drawer forever. And it's free.
So then overall, most shots of 35mm are about $4.00 (tri-X, etc.) / 36 = 11 cents for film + maybe 1 cent for developing = 12 cents a shot. Vast majority of cost being the film itself. (full frame digital is around 2 cents a shot)