$100 is a little tight to do it in, for a "decent" kit.
Can do a "decent" basic kit in $150. Once you get below $150 things like quality control, longevity and performance tend to get sacrificed.
$49 - Eheim 1048 or DTek Mag 12v pump
$43 - Swiftech MCW6002, or Dtek White Water
$44 - Dtek Pro Rad Shroud Combo, or Black Ice Pro Radiator with shroud
$14 - Tubing + radiator fan
--------
$150 + postage
To get to $100, and still be somewhat decent, you'll have to do some DIY:
$35 - Hydor L20 pump
$25 - DTek Customs Poly-topped TC-4 waterblock
$25 - Car heatercore + whatever it takes to put fittings on it for you (~$5)
$10 - Tubing (vinyl) + radiator fan
--------
$100 + postage
Could go cheaper again on the pump, but then you're starting to get into real cheap'n'nasty territory.
25USD 300gphpump: This is way quieter then I thought it would be. In my case, sitting on a pad it is about as loud as stock ATI vid card cooling. You can find it cheaper if you look around. Its high flow will let you look at some bigger rads that you might find.
I also used the swiftech block.
Find a rad on ebay. Search for oil cooler, trans cooler, heater core, AC condensor. Make sure it is new. If you search you may even find one that you wont need to put fittings on. I wanted the biggest one that would fit in my case. Its like 10x7x2.5.
If that sounds like too much work get the budget bundled one. Im trying to convice my buddy to make a move on one of those.