my $700 Mac Mini has a 10gb NIC. 2.5gb and 5gb NICs are very common on modern PC motherboards. Modern servers from Dell and HP are shipping with 25gb or even 100gb NICs.
The cost of 10g is much higher than a single computer. The entire networking stack must be upgraded to 10g. At the very least the Internet device, and possibly the Internet connection as well. It will be cheaper in the cloud than on site.
Well, it depends on what your use case for "10g" is. If all you care about is fast file transfers between your PC and your NAS, you can get a small 5-8 port 10gb switch for under $300 that will easily handle line-rate traffic (at least for large packet sizes)
If you want 10g line-rate bandwidth between hundreds or thousands of servers? Yeah, I used to help build those fabrics at Google. It's not cheap or easy.
10g to the internet is more about aggregate bandwidth for a bunch of clients than throughput to any single client. Except for very specialized use cases you're going to have a hard time pushing anywhere close to 10g over the internet with a single client.