Just my .02.
I had a MH/PC fixture on a 180 (72" x 24" x 24") that was in need of lamp replacement, I priced it out at $350.00. So, I was in the same boat then as you are now. I think the LED's fixtures are about where Plasma and LCD TV's were a few years ago, really nice products, but still too pricy. So, I bought two of these:
http://www.ebay.com/itm/Aquarium-Co...644?pt=LH_DefaultDomain_0&hash=item3f0bcd83cc
for about the same price that it was going to cost me to replace the lamps in my old fixture. Although I don't have the ability to dim, I can control the on/off of the blue/white, and I'm exhibiting good growth and PE on my stoney's and have both Maxima and Crocea Clams in the substrate. So, I'm looking at these as an interim fixture, that I can realize many of the benefits of using LED's without the huge initial cost. And when the better, more controllable units come down in price, I will revisit this issue.