Are you doing this yourself or having it done? I did all my own windows, and if you're semi-handy with tools it's pretty easy to do.
Two things I didn't see mentioned is what kind of window (picture, crankout, doublehung) it is, and what kind of siding you have.
My old windows were the integral brickmold/wood sash type. I took them out to rough opening, cut back the siding slightly to clear for the nail fins, wrapped the opening with tarpaper to prevent moisture buildup, slid in the new window, nailed it down, and insulated around it with Great Stuff Window & Door expanding foam.
Towards the end I was getting to the point where I was doing a window/hour.
If you have standard rough openings, Anderson makes a decent window for not a lot of dough. They have them at Home Depot.
Marvin makes custom windows for a little more, but they are also a decent window. I have a mix of both, and the gray vinyl on both is so close that you can barely tell the difference.
I have a bunch of pictures of the progress, in you're interested.