The real reef rock branch stuff is about $16/lb retail.
I will say I recently bought a few boxes of the caribsea branch and it was very difficult to work with and the shapes were not great. I was pretty disappointed. But, with branch you will use fewer pounds of rock than if you went with traditional rocks-I'd just make sure you have a sump full of good rock to help with the biologic processes.
Here are a couple other places you may want to try:
Good Luck and I can't wait to see how it turns out.