The least cost route depends on what you already have. (He stated very obvioulsy!)
So do you have a USB "modem" for your broadband connection or a router?
If you have a router does it have more than one network cable port on it?
If in doubt post the make and model.
Memetic.
P.S. if you have a USB modem for broadband does the PC it is connected to have a network card tha tis not in use?