Hi there. I'm the developer behind Paid Memberships Pro, which is a free membership plugin in the WP plugin repository:
In the case of PMPro, you would want to create different membership levels if you wanted to lock down different content for different user groups (membership levels). There is nothing keeping you from making them all the same price. (Not sure what you mean by "equal so to speak".) Levels don't have to be progressive. They can be different content entirely.
One limitation to PMPro currently (and some of the other plugins) is that one user can only have access to one membership level at a time... which is maybe your real problem. A future update may fix this, but for the time being, this is a constraint of our plugin.
Also, not sure what you mean by "control the stream of income". Do you mean you need to use a different PayPal Account or Authorize.net Account for each level? That's not possible with any solution that I know of. Our plugin does pass along information about the level, so you can do reporting in your gateway to get all sales for level X, etc. In general though, the money all goes into one bank account and you'd have to do reporting at some point to separate things.
I think that multi-site might be a good solution for you. You can install PMPro on each site individually, and then have a different level and/or payment gateway for each site.
If multi-site would be tedious, I would look for a plugin that support multiple memberships per user... or even the membership addons for some of the ecommerce plugins. Or wait for PMPro to add this. :)