What is the best way to share an external drive for backing up 2 Macs?

I have an USB 500gb drive, that I would like to use for my two Macs.

I know that Airport Extreme is one solution but it is quite expensive and some of the other routers I checked (Belkin N+) with an USB port, do not support Time Machine.

Is Airport Extreme the only solution?

Thank you.

You could enable sharing on Mac A, and then connect to the drive from Mac B. I don't know what you can do to keep the drive mounted on Mac B over time, but I'm pretty sure this is the beginning of a solution.

This is another problem that I just discovered. Mac A can connect to Mac B wirelessly and see the files but for whatever reason Mac B cannot connect to Mac A.