Select the FAX machines you are interested in, and then explore to find the underlying printer engine.
Costs per page comparisons of printers is easy enough to find.
Use any such comparison as a very rough guide, as you could be recieving photographs or other images requiring quite a bit of toner or ink.
If you are not a heavy user of fax inbound, avoid ink-jet based fax printer engines and use a laser-based engine.