Shnoop via eBay offers the Oral-B Power Sensitive Extra Soft Replacement Electric Toothbrush Head 3-Pack, model no. EB17-3 ES, for $13.99 with free shipping. That's the lowest total price we could find by $5, outside of the mention below. It's compatible with all Oral-B rechargeable handles, except for Oral-B Sonic toothbrushes.

A close price: Amazon has it for $15.19 with free shipping via its Subscribe & Save program.