Amazon offers Prime members the Sony 64GB UHS-1 Class 10 SDXC Memory Card, model no. SF64UY2/TQ, for $14.39 with free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $8.) Outside of the mention below, that's the lowest total price we could find by $6, although we saw it for a buck less last month. It features transfer speeds up to 70MB/s.

A close price: Focus Camera via eBay has it for $14.99 with free shipping.