In New Leaf you can now rent museum space on the second floor for 10,000 bells. However the wording "rent" made me wonder; is this a recurring payment or does 10,00 bells cover me until I decide to retire the exhibit? If it recurs, how does that work?
I am pretty confident that it is a one time charge.
It is not typical of Animal Crossing to ever "bill" you any number of bells automatically. Also Celeste only mentions putting items in the locker if you opt to retire the exhibit.
I've had the exhibit for several days. I have not been charged any extra amount of bells. I have also talked to Celeste every day and she has never mentioned anything.