like a house window?
i clean windows for a bumming during summer and get asked this question all the time.
it depends on your local prices really. and if you need any frame work which most likely you do since most windows cant really be repaired. its cheaper now-a-days to just get the whole thing done, frame and all. a single pane window though? sounds like a pretty basic framework. what is it? door, sliding door, pull out window etc?
pella is super easy. actually theyre all really easy since theyre double hung they just pop out.
youd have to check with a certified dealer though cause i can only look up like a national average. generally around 300 bucks. but a 20"x20" (inches right?) is rather small, so maybe more like 200.