Cheapest harness for radio

Hey guys/gals. I have debated on upgrading my stereo for quite some time now. I have the 2005 GTP with the Monsoon system. I am wanting to add a deck and possibly a sub with amp. Now I have heard there is a harness that will allow my Monsoon to still work, plus all my chimes and such. This harness is super expensive. I was just wondering if you guys knew of a place cheaper. Ive tried all the local audio shops. Maybe it's just expensive in Canada.

look on ebay for PAC interfaces. they run from 49 to 100 $. also check amazon

Thanks dude. Thats a big help because they are a lot cheaper. Ill have to research it a little more because they don't list for Monsoon or Pontiac.