Journal of Computer Graphics Techniques recently published a breakdown of their costs hosting an open journal. Approx. $100 per annum ;)
However, even if you were to pay the people behind the journal for their work it should be fairly inexpensive as long as they did as little as Elsevier does.
(Disclaimer -- I'm co-author on a paper published at JCGT and have been a collaborator with a few members of its editorial board. It's still a highlight of my career have published with Tom Duff of "Duff's Device" fame and had the paper get a nice twitter mention from John Carmack: https://twitter.com/ID_AA_Carmack/status/846525438797086720)