Are they putting anything else up there besides the blown in insulation?
We paid about $1200 for blown in insulation & aluminum barriers between the split roof (to reduce heat transfer).
Also, check if they'll put in soffit vents if you have soffits. I forgot to ask and they didn't. They did take pictures to make sure soffits weren't covered over (I could see daylight coming through), but I'd much preferred to have real vents.