Adorama offers the Lexar Professional 400x 128GB SDXC UHS-I Class 10 Flash Memory Card, model no. LSD128CTBNA400, for $49.95 with free shipping. (B&H Photo Video charges the same.) Aside from the mention below, that's the lowest total price we could find by $31. It features a reading speed of at least 60MB/sec.
A close price: Amazon offers it for $50.99 with free shipping.