I just completed an audit of my company's fax servers. We have to maintain incoming lines for clients that we do outsourcing work for.

The perform the review I needed to send a 400+ faxes to all kinds of numbers. Used Interfax.net for the work, and found them inexpensive and with an efficient user interface. Their pricing system is structured around buying a prepaid allotment of pages. I think $10 is the minimum spend.