Comparing window quotes on the same basis
Window quotes are hard to compare because two installers can be pricing genuinely different jobs. The first thing to pin down is whether each quote is an insert (pocket) replacement, which reuses the existing frame and is quicker, or a full-frame replacement that strips back to the rough opening and lets the installer see and fix the sill. The second is the spec of the unit itself: frame material, the number of panes, whether there is a low-emissivity coating and gas fill, and the warranty term on the glass seal as opposed to the frame. Ask for the manufacturer's model line and the exact opening sizes on the quote, so that you are comparing the same window at three prices rather than three different windows.
Then check what sits around the window. Interior casing and sill, exterior capping or trim wrap, caulking and insulating the gap, hauling away the old units, and any permit or heritage approval are all things that appear on some quotes and not others. If the house is older, expect the possibility of rot in the sill or framing once the old unit is out, and agree how that will be priced before it is found. Get the lead time in writing as well, since ordered windows are made to size and a delay can leave openings boarded up. Hold a contingency of roughly 10 to 15 percent, and be wary of a price that is only valid if you sign today.