First of all, you do not need ABS. Many people prefer to drive without it. This especially true about ABS designed in the 90's, which weren't all that great even when it worked "welll". The trick is in being able drive it now like a non-ABS car!

Second, any mechanic who is ofering to fix ABS for $4000 on a car that is worth $6000-$8000, is not looking out his customer's best interest. Most mechanics are aware about cheap rebuild options for many electronic components. Good mechanics will offer it to you, lazy mechanics will not want to wait for the rebuilt cycle (or deal with redoing the job if the rebuild unit fails) and they would offer the crazy, new OEM price! I would stay away from such lazy mechanics...

As for a cheaper solution, this forum has everything you need to know to come up with it