For Prime members only, Amazon offers the Lexar 633x 64GB High-Performance microSDXC Flash Memory Card, model no. LSDMI64GBBNL633A, for $15.99 with free shipping. That's the lowest total price we could find by $2. It features transfer speeds of up to 95MB/s and includes an adapter.

Update: Note that it's expected to ship in one to two months.