Seeing as I'm mid retrofit of Xenons myself Rich, these are just estimates from when I called Parkway Northampton previously but;
Per Xenon headlight- £510 inc vat ish
Ballasts are about £220 inc vat (each)
Bulbs about £130 inc vat (each)
Ignition starter not sure as I didnt ask. Bear in mind this is what I can remember from a rough post it note from over 6 months ago.
Ebay is the way to go for a set as you may well be aware as they come up frequently in good nick with bulbs + ballasts.
HTH