Amazon offers the Corsair 8GB (2x4GB) PC3-10666 DDR3-1333 SO-DIMM 204-Pin Dual Channel Notebook Memory Kit, model no. CMSO8GX3M2A1333C9, for $59.99 with free shipping. The $10 mail-in rebate cuts it to $49.99.
The 8GB (2 X 4GB) DDR3 Laptop Memory Kit is 1333MHz Unbuffered CL 9 SODIMM Memory 9-9-9-24 1.5V which matches recent iMac and MacBook Pro Specs. Commenters at Amazon concur.
Apple charges 4X that for an 8GB upgrade: