There's an inexpensive HID low-beam module on the page I linked above, it does not come with the HID capsule or ballast. You could probably find OEM ballasts and HID capsules elsewhere and have a much cheaper system that would still maintain safety.
Yup!
Ballasts and bulbs tend to be very overpriced when buying brand new, or from a dealer.
It's easy to find a pair of 35W D2 ballasts pulled out from a recent car for under $100 on ebay.
Brand new Philips 85122 bulbs are only about $70 per pair on ebay as well (people buy/import them in huge quantities, then sell them in pairs at much lower prices than dealers and most retailers)
Most have similar performance. Watch out for waterproofing. They should be sealed very well. Or, consider purchasing Matsushita Gen 3 ballasts (pull-out from many newer Nissan and Infiniti models). They are waterproof except for one connector on the top, which makes sealing a lot easier. They're designed to be inserted into a ~3" hole in the bottom of the headlight, and sealing against it, with the majority of the ballast exposed to the elements, whereas most ballasts are designed to be fully enclosed within the headlight, making them difficult to mount in custom applications. I just sold a pair last week. With the pigtails and wiring included, they're usually $90-$125 per pair.
I'd avoid buying used bulbs if I were you, because you have no idea how many hours are on them. With BNIB bulbs for $70 a pair, why buy used?
All in all, a fully legal Hella 90mm HID setup would be under $500, not considering the relay harness and housing. If you're brave, you can buy projector assemblies pulled out of another vehicle, but then you would need to fabricate your own mounting system, housing, adjustment brackets, and sealing. The Hella unit is just four screws into the front of the car, and it's already in a watertight housing.