I have a Trek 24" T-5 6 bulb setup that I'll be selling soon but I think that to large for a Refug. Check with HD that have single T-5 bulb setup, I saw one last week while I was in one of their stores, that was pretty cheap and would do the job.
check out http://www.aquatraders.com/index.asp?PageAction=VIEWPROD&ProdID=12
At $150, it would have t be the cheapest metal halide around. There is a huge thread on RC about them (23 pages and still going) It would seem most are happy with them but there has been some issues with stuff being broken in shipping. Also someone mentioned that the supplied legs dont fit a 24" tank like the description says, but if you're going to hang it then its no big deal.
I am actually thinking of getting this setup for my 26G bow
Aaron