Well if your computers are near each other and you dont need a wireless set up you can indeed save money and get a router quite cheaply.
A Linksys wired router can be picked up for about £30-40.
You mentioned you had ethernet cards in both machines.
All you have to do is hook up a lead from each PC's ethernet cards into the router and then plug in your cable modem into your router using an ethernet cable, and not USB.
Then its a simple process of setting up and your away.
Check this one here:
Linksys Wired Router