Is $30,000 to replace half or potentially the entire roof a fair price?

I am in escrow on a house, and half of the roof needs to be replaced. There is a power purchase agreement with SunPower that states Semper Solaris is the only company allowed to install/remove the solar panels. Semper Solaris said they would not remove the panels for another company to do the roofing work and that I would need them to replace the roof. They will not come out to give a formal quote until I actually own the house, which means I can't go to the sellers for concessions based on their price. I received this ballpark estimate over the phone. I have gotten quotes from a few other roofing companies for the work that needs to be done, and they range from $12,000 to $16,000. I am wondering if Semper Solaris is really that much more expensive than other roofers.