Minimum: Obviously 0. There are people who supply free housing.
Maximum: This can very depending on the size of your house and the services. Also. Location. Location. Location. I would say between 200-1000 rc weekly or bi-weekly (every two weeks). 200 would be an 8x8 prebuilt house and 1000 would be a 20x20 lot where they can build. Also depending on how close you are to spawn

.
How big would you want your room or house: In New Aquaria I have an 8x8 lot fully custom. It fits my needs. ( I pay nothing for it because I paid my dues being a builder). If I had to pay 300-400 every week would be fine. Essentially a 20x20 residence in a town is a MANSION of sorts. That is a very large area for a residence. 1000 rc a week is fair.
How many credits would you pay if you were to stay in the realm forever (or untill you moved out) with a one-time payment: This is completely dependent on if you want to double your credits or just float. This also depends on how many lots are on the realm. If you have 16, 10x10 lots, just to pass bye and get your 10k back they would pay 625rc one time. If you wanted to double your cash 1250rc one time.
How many people would actually use this service: I can't really answer this for you. I believe it is sort of exponential. The more people you have, the more people you can get.
I added one more:
Services the Land Lord should provide:
Furnace room: A place for people to smelt their materials. ( Do not leave unattended)
Bank: Safe storing on accessible by 2 or 3 of the most trustworthy players.
XP Spawner: If you have a spawner make a multi person XP spawner.
That is about all I can give you so. Yeah.