I actually had a small local audio shop quote me for the sub enclosure and he came up with an estimate of 2700$ (for just one side!) Apparently custom fiberglass enclosures are freaking expensive.

I recently ordered the monsoon sub enclosure from gmpartsdirect and I'm going to have him install that instead. Much cheaper! Although he hasn't given me an estimate for it yet. Heres the threads that talk about the adding the monsoon enclosure:

Part Number Needed for stock Monsoon Sub Woofer Enclosure

*HOW-TO* Install factory subwoofer in non-monsoon car