just wanna know how much it would cost to convert the rear drum breaks on the xb to disc brakes.

There are no Kits you can buy to do this.
Therefor the price depends on your mechanical abilities scrounging skills and how often you dorp by your local shop to run for pizza as a "courtesy" when they are working late.

Slowpoke21m did a rear conversion fo a bout 100 dollars, but it wa s a DIY effort

http://www.scionlife.com/forums/viewtopic.php?t=2670&highlight=rear+brake