As Whiskey noted, there are companies that can scam you big-time. Find a carpenter or contractor that you can trust for the actual replacement labor. Ask your banker or building supply store for the names of reputable builders/contractors; they usually know who the best people are. Most window companies offer a 10 year warranty on the glass, but labor for replacement is usually not covered. I have hundreds of aluminum windows that are still in good shape after 30 years. If you order aluminum replacement units, be sure to order the units with a thermal break to prevent sweating. I've also installed hundreds of all vinyl units with equally good success, and I've seen no deterioration in the last 20 years. If you have the option, select vinyl windows that have foam-filled frames for extra insulation. I'm currently building a home for my daughter, and after reviewing prices and performance, I decided to buy Andersen "builder model" all-vinyl windows (can't remember the series #) because Andersen has been around a long time, has a good reputation, and also has a good warranty. I ordered them with Low-E glass, foam-filled frames, and wood extension jambs, and the price was quite reasonable. When replacing windows, you will need to order them without nailing fins because nailing fins are only used on new construction. If the window units are very big (which they're probably not on your home), you will need to order glass with higher impact resistance because of wind pressure. I'm guessing that a good carpenter can replace your windows for $75 or less per opening, but that's only for the replacement labor. You would still need to hire someone to paint the extension jambs and reinstalled trim and caulk the exterior joints. A handyman/carpenter can probably do all of the work for $100 or less per opening. Before ordering replacement windows be sure to get someone with experience to look at your existing windows and how they're installed so that you order windows with the correct dimensions.