Collecting payments when the price constantly changes

I have a customer who would like to charge his clients through the website but each month the clients are billed for different amounts so I wasn’t sure how to set it up. Is it possible to have his customers set up accounts and then bill each of them the correct amount each month? Ideally they would be sent a email to pay that includes a link to their individual account where they can view their balance and then pay right there on the website.

If anybody has done this before and could point me in the right direction for a tutorial video or plugin that would be awesome. I’m really open to ideas here, i’m not sure what the easiest way to do this is.