You can get the HelloFax for Google Docs add-on for free in the Google Docs add-on store.
With a free HelloFax account, you receive 5 free fax pages, with a chance to earn up to 25 pages total while setting up your account on our "Getting Started" page. After that, it costs 99 cents to send a fax of up to 10 pages, then 20 cents for each additional page after the tenth. So for instance, a 10 page fax would cost 99 cents, and an 11 page fax would cost $1.19. With the free version of HelloFax, you can send outbound faxes only.
Our paid HelloFax plans include an inbound number so that you may receive faxes, as well as a monthly recurring allotment of fax pages, depending on the plan you choose.