There are a couple options for aftermarket HIDs and I will put them in order of what I would recommend.
Full OEM conversion, we take OEM HID headlights, ignitors, and bulbs and replace your non-HID headlights with them. This is the best way to do it and usually costs in the $2k range. This is not a garage mechanic friendly job, we dont even like to do it unless we are doing a few other things at the same time.
Use of existing headlights with OEM bulbs, and ignitors. This is the next best, it typically can be done for under $1k but I only recommend it if you have projector headlights already. This is also a pain in the ass and we dont really like to do it.
The 3rd best option is what I normally recommend and what I have done on my cars, a plug and play aftermarket kit. These come with a lifetime replacement on the bulb and ignitor and can be installed in an afternoon. We do them for about $700 installed and in many cases these offer the best bang for the buck results.