Normal call charges apply.
My fax (using Win XP fax program) is on my normal phone line and I've signed up to one of those pay so much per month for local and national calls so I don't actually pay anything for sending faxes, just the monthly fee. Receiving a fax costs nothing because the sender pays.
If you've got a non-geogrpahical number (0845, 0800, 0870, etc) then it's still normal charges apply as if it was a phone call.