The cost of hosting a website depends on the package you get frm the hosting provider.
In broad terms you will be charging depends on the following:
Number of email addresses available
Number of domains to be hosted
Number of sub domains required
Shared or dedicated servers
and a few others...
I don't know of any company that limits the number of pages you can have, although anyone is welcome to point one out to me!
So, the number of pages you have is likely to be inconsequential as long as you don't go over your storage allowance.
There are companies that offer unlimited storage and bandwidth, which may be suitable for you depending on exactly what you want from you site.