I could say "oh, a few hundred dollars" but in reality it depends hugely on the details of your situation.
If you can find someone who's good with YOU doing the trenching and cutting the holes in the walls and running the line, and they just come in and do the connections, that's a lot cheaper. If your municipality doesn't require an electrical permit, that's cheaper (but if they DO, I would encourage you to pony up for it rather than sneak around, for various reasons).
OTOH if it turns out that the circuit you wanted to put it on is already pretty much 'full up' to capacity, or anything else like that, requiring significant rewiring in the house to get power TO where you want to put the coop line, expenses can and will add up.
It sure is worthwhile having proper, correctly-installed electric outlets available in the coop, though!
Best of luck,
Pat