It's more or less all speculation at this point. It will have to be competitive with hair transplantation price-wise though. hair transplants can give a NW5 a reasonably full head of hair for ~$20000-$30000, so HM probably won't cost a whole lot more than that.
I think though, that once they get it perfected and its been out for a while, it will cost less than hair transplants do, because the actual procedure is less complicated and invasive. Taking the sample and culturing the cells will consume more time on a per patient basis, but hundreds or thousands of patients can be done simultaneously, and the culturing would be done by lab techs whose salaries are much lower than an MD's is. The injection process should take less time and effort on the part of the doctor than transplants too. I imagine dermatologists and regular plastic surgeons could do HM injections, without being as specialized as regular hair transplant doctors are now.
Of course, this is really all just my opinion. I don't have any idea what may happen. Costs may be outrageous initially, because a lot of very desperate, very bald men might be willing to pay a lot of money for the procedure, which will initially be done by only a few doctors, but once a lot of doctors are doing it, and a lot of patients have had it done, prices should be reasonable.