Intel 10G NICs are $300usd a pop so 10G is definitely the more expensive solution here.
Used Chelsio 10G NICs are dirt cheap, though, like
this one for $30 (though it's admittedly a very old model). A more-recent unit will be more expensive, but still can be had for about $100, like
this. Chelsio is exceedingly well-supported in FreeNAS. You don't need a 10G switch, either--just a patch cable from the one to the other.
In any case, as we've already discussed, no standard PC is going to have the proper hardware (a USB B port) to do what you're looking for, and FreeNAS doesn't support it. The only supported way to access your data is over a network.