iFax offers three monthly plans beginning at $8.33 for 200 pages. You need the Pro Plan, which costs $25 per month (billed annually) to access the developer APIs. This plan already has 1000 pages and several advanced faxing features like fax-to-text OCR conversions, fax by email, and dedicated mobile apps. It also includes a signed BAA for HIPAA-compliant faxing. With the developer APIs, you can configure fax integrations to fit your workflows.
WestFax also offers three monthly HIPAA-compliant plans starting at $14.99 for 500 pages. API access is available with their $39.95 growth plan with 1,500 pages for five users. Each excess page will cost you 3¢.