10G Ethernet is available in PCIe expansion cards but then you need to factor the cost of a Thunderbolt expansion chassis. There is also a stand-alone Thunderbolt to 10Ge available:

http://www.attotech.com/products/category.php?id=15&catid=16

You can get it for ~$800. That might seem expensive but the first 10G to PCIe card I bought from Myricom four years ago cost me $900. Now it is ~$300.

I can only hope that Thunderbolt catches on so we can see peripheral prices come down.