Please correct me if I'm wrong,

Currently the network hashrate is 17.35. Amazon EC2 has a product that have 2 x NVIDIA Tesla M2050 GPUs. This have a combined power of 160Mhash/s. 17,350,000 / 160 = 108,000 instances to achieve 51% attack.

So 108,000 * 2.10 = $226,800 / hour to achieve 51% attack. Is my calculations correct?