I just don't see the logic. If you have, say, 100 people who need your help, but you can only help 10 of them, why do you choose the richest ten?
I understand if your primary goal is to make money, you should definitely pick the people who will pay you the most. But if you really want to help people, surely, you should choose the ten people who need your help the most. And if that's your goal, you might do better to pick the ten who are
least able to pay you.
Alternatively, you could have a lottery system. At least in that case, the various spirit guides involved could arrange for the neediest people to win the lottery...?
Frankly, I think the chaotic system you're using now (which isn't far from a lottery) is better than choosing people based on their wealth.
I understand you have a goal of making your site ad-free, but honestly I don't think this is a great way to do it.
I offer a lot of downloads and services on my site, and everything is available for a donation of any amount. I've had a lot of success with it so far (though I don't have anywhere near the volume you do). You can read more about the rationale
here and
here.