At a rought guide expect to pay from approx £4.50 to £5.00 for live rock from a reefer breaking down a tank - if it's any cheaper than look for what the problem is - covered in aiptaisia etc. but a good reefer would tell you that!
I don't remember seeing any good reefer selling good live rock for that sort of price, so buyer beware - do not part with any money until you have actually seen the rock in the flesh, so to speak. Most deals that are too good to be true are just that, too good to be true!!!!
Seffie x