For 250 per window you would be
lucky if it was actually junk at that price.
The contractor attempting to con you into buying $250 windows should immediately be eliminated from consideration.
High quality brands would be HiMark, Soft lite, polaris ultraweld,okna.
There is a well respected company in your area called " HomeSealed ".
At the very least i would at least talk with them.

+1

You need to check out HomeSealed. He is the man in that area and you will not find a more educated, consistent, and considerate contractor.

Post back up once you have spoken with him and I guarantee that you will be much more at ease with the process.

There are a lot of reasons for the discrepancy. Quality of product, installation, type of installation, size and business model of the installer, etc. The key is to try and make an apples to apples comparison on all of the main points. A good price on a good product will be somewhere between those price points, admittedly closer to the high side.