I don't follow. Did they want $900 for one but only $300 for the other? Keep in mind that there could be a lot of differences in how the head was rebuilt. One may have only been tanked, had new stem seals installed and springs tested. While the other may have had new springs installed, new seals, been ported and polished, had a valve job and so on.
Ask for some details. There might have been a legit reason for the difference... or they may have been trying to screw you.