Is $17,000 to install flooring for 1500 sq ft a fair price?

I'm getting my floors replaced in my 1500 sq ft house. I buy the materials, and the contractor charges for installation. The initial quote was $17,000, but halfway through the project, I received a change order increasing the install cost to $30,000. I understand that some prep work might be more than anticipated because my house is a custom build, and everything is unexpected. However, the contractor is charging me for things like miscounted steps and patterning that they knew about ahead of time, which was even in the original quote with pictures. They are also charging $125/hr for grinding concrete. I feel like the contractor is scamming me, as $20/sq foot for just installation is robbery and not what was promised. I had a conversation with the contractor where he admitted miscounting the stairs and double-charging for patterns. He even suggested I could hire another company for the stairs or pay the extra. I believe a miscount is not my problem, and I shouldn't have to pay for extras I didn't ask for. I also don't think the installers should suffer because of the contractor's mistakes.