I have replaced thousands of windows, put myself through college because of siding, windows, and decks. I still install about a dozen a year for our vacation fund. I am adamant about using vinyl replacement windows. Wood rots, period.
Have the person that is going to install your windows, measure and order the windows. Asking for trouble when a salesman does it and then has subcontractors install, in my opinion/experience.
A good standard sized window can run between $175-$350, depending on what bells and whistles you desire. Install with insulation, caulking and reusing trim on the inside runs $75-$150 depending on type of old windows (window weights, brick, stucco). I used to charge $50 a window to cap the outside with aluminum that I bent with my break, but aluminum prices went up and now I charge $70. So depending on size, on most standard houses, I can tell the owners that I can replace their windows for about $400 a hole if they order quality windows without lots of fancy (like blinds between panes).
Lots of options, but don't just jump into a deal with the first pella or Anderson salesman that comes to your house saying that they are going to give a "deal" and knock $400 a window off and go from $1250 to $850... Don't fall for that crap. Get a contractor..... Most love to have a side job that will take three days (a couple of weekends) and still put a grand in their pocket.
