Amazon offers Prime members the PNY 64GB Elite Performance UHS-1 Class 10 Secure Digital Extended Capacity Card, model no. P-SDX64U1H-GE, for $26.99 with free shipping. (Non-members can spend $35 or more to bag free shipping; otherwise, it adds $8.) That's the lowest total price we could find, outside of the mention below, by $6, although we saw it for $3 less in February. It features read speeds of up to 90MB/s.

A close price: B&H Photo Video has it for a buck more.