The PCI-e 16x version is as low as $63, but you cannot use it.
If I were you I would bite the bullet and save up money for a new PC. You have very limited RAM and spending $104 to upgrade it is not really worth it. You will see some gaming improvement, but overall you will be limited to playing older games (circa 2004 - 2005 at best).
For upgrading a machine that old it usually is best to start new. If you let us know what your budget is we can help you build a good one that can be upgraded down the line, to save on money you can atleast reuse the optical drive and the HDD(but you need to reinstall the OS) if you need to keep costs down.