Depends on the ballast you currently have. Some are incased in a metal box which can be a pain. Others you just swap right out. Post the make and model or a picture or two.
I would try contacting the manufactorer to see how much a replacement is. Otherwise hit mufret up as he is pretty much the king of manually repairing these things. It could be something as simple as a fuse.
too late--it went for like $130--it was a current 3X175 watt fixture w/ 4-096 watt PC's --I didn't really want/need it, but for a couple hundred bucks thought I might find some use for it ---but I didn't win it