In nj and that is too cheap. Compare windowmart to homedepot. Homedepot will get you contractors who will work for the lowest cost possible using products from homedepot. That means slap it on, tape it, wrap it and call it finished. Homedepot vinyl windows (the lowest price - american something brand) is junk.
Even if you went to andersen windows, their replacement windows may not be good windows. Andersen has replacement windows branch which seems to be selling very loose sash double hungs. Get to know about windows.
Is mastic a brand of vinyl siding? If yes, it may be a good brand, previously known as something else with some history. Sorry do your research to confirm. It still doesn't do you any good if the install is sloppy and leaks into the house. Almost ALL vinyl siding (it looks like) is installed to leak into the house and depends on the wrap and some flashing to keep the water off the sheathing and indoors. Even youtube pros do this with the j channel arrangement. My belief only, but there should be a limit on how much water is let in, but most vinyl siders ignore this.
Also how did you get this quote? Did somebody come to your house and measure things? How about your area and a scan of the contract? Make sure the contract has ALL of the details and the forum can help you with such things.