Absolutely no way anyone can give you any sort of an idea from the info your providing.

+1. Very hard to say... What I can say, is that it would not be very easy (and probably not to appealing) to add a sidelight to the existing entry door that is already installed. What style door do you have? If you are just looking for additional light, you could consider a door with more glass.... Otherwise a window "cut-in" to the foyer area might be the best option. With no further info, a total ballpark number on what something like that would cost from a reputable company would be a BARE minimum of $1k-$1500 or so, but likely more. Considerations include: material, size, and options of window, type of construction, exterior cladding type, any electrical plumbing or hvac in the way, etc, etc.