You dont need to have the water system contained elsewhere, you can have it right in your computer. Go to www.dangerden.com
and peice together your own, that's what I did. I have a $20 heatercore that I modded on my own, put some barbs on it, and it is nice. It's not terribly big, but will do the job quite nicely.
If you are finding water cooling kits for $500 up only, you are either looking in the completely wrong place, or at the completely wrong thing. The average kit is, like stated above, $150-$200. Also, like stated above, do NOT cheap out on water cooling! Unless you ONLY want water cooling for the bling, to be able to tell peopel you have it, than be my guest and cheap out and get a kit. You will get better temps with stock air cooling that you will with you average kit. The only kits worthwhile are the ones at dangerden, because they have good parts in it. It is what most people would call a better than average water cooling solution. It's just that dangerden took soem really good parts, sold them as a kit, and charged more money for it. If you dont know what to get, and are determined to get a kit, dangerden should be your ONLY option!