I bought a used ballast for my 04 facelift which uses a different ballast than yours, got it for around $80 IIRC. But I saw ones like yours for pretty much the same at the time. A brand new one was about $235 then.
Get the part number of your ballast over at
BMW Parts Catalog then plug it in on ebay, you should get a ton of hits. IIRC a few different brands of cars actually used that particular ballast so it's fairly common.
I just checked and it seems there are adapters to mate your factory D2 plug with an aftermarket halogen based hid bulb (Amp to D2 adapter). So you can buy factory ballasts and get your factory lights working now, and if you get AE assemblies in the future you can buy rebased capsules and the adapter and make it work, or replace the AE projectors with a D2S ones and just transfer the stock bulb and plug in the stock ballast.