I think I only pay like $5 or $10 for what they call a "teen line". Basically I have one single phone line, but two phone numbers, when someone dials my fax number it uses "distinctive ring", which is kind of self describing, a different style of ring is sent to the phone. I use a device I bought online for about $50 to send the distinctive ring to my fax computer, otherwise it forwards the caller to my regular phone.
While my sign business doesn't use the fax a whole lot, my business partner has all his faxes for his other businesses sent to us and he receives quite a few (mostly from his towing company, which they fax PO information/customer info/etc). I really like having a computer handle all the fax receiving, as sometimes faxes get lost in the shuffle, but the computer keeps em indefinitely, you can also view it beforehand so you don't waste ink/toner if you ended up on some ad list. Only downside to my setup is I can't call someone and send/receive a fax at the same time, but that rarely happens.