First off, you're right, if you want a router and you've only got 2 pcs, a wireless solution may not cost that much more than an HPNA solution. HPNA may be cheaper as you add more pcs; also, wireless doesn't always work well in all locations, e.g. in some big houses with the pcs far apart you might have problems.
Unfortunately, if I understand what you are proposing, I don't think it it will work. If you don't have a NIC in either machine, then the USB to ethernet adapter will make it possible to hook up one of your machines to your router. But, you still have the problem of the other machine; without a hpna/ethernet bridge, the router can't communicate with it. Maybe you could get something to work if both machines also had hpna cards and the machine connected to the router was always on, but then I'm not sure what the point of getting the router is (unless it is maybe to provide some added security).
I can't vouch for it, but here is the cheapest hpna/ethernet bridge I have seen, only $72:
TH102 from Buy.com
Buy.com also has a 1-day special on the Netgear mr314 router with built in WAP. Buy that, a NIC (or, if you prefer, your usb to ethernet adapter) and a wireless card or usb device for your other machine, and you'd have a complete wireless setup for well under $200.
So in short, if you want super-cheap, your hpna kit is the way to go. If you want a router, in a 2 pc setup I think the costs of wireless and hpna won't be that much different. You might go super-cheap for now, and if you don't like it then in 6 months get wireless when it is cheaper and/or faster. Maybe hpna 3.0 devices will be out by then too (hpna 3.0 will supposedly be 10 times faster).