Hi Lol,
OK, here's a quick attempt at a calculation (If I've done any sums wrong, hopefully someone will point it out)...
I currently use HC-110 and Tetenal rapid fixer. The last HC-110 I bought cost approx £9 for a 500ml bottle of the "European" concentrate - weaker than the syrupy stuff that is sold in the US (which can also be bought here in the UK - I'll be buying it that way next). I use that at dilution H, which is 1 in 20 using the less syrupy stuff. That makes 10 litres of working solution from a 500ml bottle, and if I use 300ml per 35mm film that's 33 rolls of film. If we assume some wastage and call it 30 rolls, that's 30p per roll for developer.
Tetenal costs approx £7 per litre, which will make 5L of working solution at 1+4. It varies, but I reckon 1 litre of working solution will fix about 10 films before exhaustion, so that's 50 films per bottle - 14p per film. Actually, my 10 films per litre is probably the best I've had, so the average will be a bit less than that. So call it 20p per film, tops.
My stop bath (water) and my hypo-clear (water) are both free (well, I pay water rates, but it's not metered), and my wetting agent (diluted Fairy Liquid) is so cheap there's no need to count it.
So I reckon it costs me less than 50p per film. And that will be reduced in the future, because now that I'm really happy with it I'll be buying the super-syrupy HC-110 next time, and that's even cheaper per film.
Developing more than one roll at a time is economical in terms of time only - I still need 300ml of developer and the same of fixer per film. But the time economy is well worth it for me - unless I'm really in a hurry for some shots, I rarely develop fewer than three films at a time (though it's a trade-off against patience - I haven't managed to wait for enough films to fill my 8-reel tank yet)