My costs so far for the p46 headlights with HID kit (3x bulbs, 2x ballasts): $0
LEDs for shroud project: I had LEDs from my interior LED conversion.
2x RX330 projectors: 1 was a gift from a friend, the other was purchased off ebay for $45, but normally will cost anywhere from $80-$160 per pair.
2x Slim 35w ballasts: $25
2x D2S bulbs: $0, reusing my old/original D2S bulbs off of the car. A new pair of Philips 85122+ will cost $95 (or $50 if you get "slightly used" like I did).
So, if you're lucky and you happen to be an engineering student, you will also have amassed a good bit of extra stuff like wires, soldering supplies, and hand/power tools (most important one: Dremel). Otherwise, you will need to invest in this stuff.
My total cost for this project is $25 so far. I still have to either make/buy a harness and get some clear lenses for the RX330 projectors. If I had halogen headlights, this is the route I would go: wait for quality parts to show up cheap, get friendly with dealership service/shop workers, look at www.hidplanet.com
for deals and knowledge.