Cost per roll, no 25 gallon purchase necessary.
I use a two reel plastic developing tank, I prefer it to my SS ones. With the reels in place, it takes 650 ml of water to cover them. As SS takes take far less liquid, any calculated costs would be that much less than my hog.
Using David's 1:15 ratio, that's 43.3 ml of stock C-41 developer to seed it with.
One liter of stock developer will give 23 loads, or 46 rolls of film.
Not including shipping, one could use the 500ml Rollei/Compard/Maco kit and it works out at $1.05 per roll.
A Unicolor 1 liter lit checks in at 41 cents, the two liter would be 36 cents.
Even the expensive ($40) Tetenal 1 liter is only 87 cents, the inexpensive-per-liter 5L kit works out to 33 cents.
Since it will cost $5 or more to either mail or drive film to a develop only facility, it's all good!