Depends on what type of well you want i.e. metal, landscape stone, concrete etc., and what type of window you want. Is the foundation poured concrete, concrete block or the old style bricks? Do you have a tile system already for the well to drain, do you want to have a dedicated sump for it, or just 12 inches of gravel 6 inches below the window i.e 18 under window ledge. Do you want a cover? If you do, most covers sold at box stores work only with the half moon metal wells. Custom wells will require a custom cover.
Have done few and the average cost for material on a basic is what Brandon just stated. It can be lower with cheaper material and of course go way up with the stuff that makes it look like a sunken deck. It's really up to you.