Thanks Steve - Brian does a lot of his own Dev, as does Pete and Hamish - maybe they can comment on costs?
I think my original set up cost me about $70.00. (Keep in mind I'm doing processing only, then scanning negs on a photo scanner.) Wait, if I factor in the photo scanner I have to add another $150.00 (Epson V500.)
I process only B&W (at the moment).
If I send a roll of B&W film out for processing (negs only, no prints or scans) it costs me about $7 or 8 dollars. I shoot a lot of film so that is a lot of money. (By the way, if I shoot color--just snapshots around town, nothing special--I take the film to Costco. They will do "processing only" if you like for $1.25 or something like that. In an hour or less.)
I use Rodinal (R09 One Shot) as my developer. It's inexpensive (about $6.00 for a 125ml bottle) and the bottle lasts a while at the dilutions I use. (Mostly 1:50 or 1:100.) It also has a long shelf life and because it is liquid its easy to mix.
Sometimes of course I have to buy other chems. Fixer is the only one I absolutely need other than developer. But, by choice and for convenience I also buy and use stop bath and hypo clear as well as a wetting agent.
I haven't done any cost/benefit analysis or anything, but I think once you get the equipment and the chems there is a significant savings to be had through DIY. Its also fun.
However, if you have access to a school darkroom and equipment I am sure there are additional savings (as well as trade-offs).