Based on my personal experience using an established home improvement company, the purchase price and installation of one standard size window can range from $300-$700 each. A bow or bay window alone can cost upwards of $4,000.
If you are considering replacing windows in your home, do a little homework first. Google "double-hung vinyl windows." Get at least three estimates from reputable home improvement companies. It's a big negotiating game once the salesperson comes to your home for a presentation. He or she will make recommendations and actually show you a sample window. Consider the best quality windows of course.
After a 1-2 hour presentation, the salesperson will quote you an astronomically high price! This is where the negotiating begins. Hold your ground. Don't be forced into overpaying with the promise of low monthly payments for 15 years.
Please note, financing huge investments like this will normally attach the loan to your house. What this means is that if you miss several payments on the new window loan, the lender has the right foreclose on your home.