Dark0ne may chime in, but maybe I can be more clear about what I meant in the meantime as I think it's still needed.
The current values of what you're talking about can already be quantified given our membership structure, as seen here:
As shown above, the only notable differences between supporter and premium status are uncapped download speeds and multi-threaded downloads. So that leaves 'no advertising' and 'supporter only image share' as features that you imply would reduce the cost by offering the download services al-la-carte. This is technically true I suppose, but consider this:
Isolated, those supporter features ('no advertising' and 'supporter only image share') are currently valued at a mere £1.29, for the life of the account. Additionally, we provide these features for free to all Premium Members, again, for the life of the account, even if Premium Membership lapses. Considering those points, I feel that makes the cost of these features rather insignificant to the cost of Premium Membership.
It's one big pay for say, well I don't know, $20 to remove the uncapped speed limit. That's just a random number I came up with, but whatever price you set it to is how well you get customers.
So essentially, I feel that what you are describing is already true: All you are effectively paying for as a premium member are the uncapped and multi-threaded downloads. Currently, you can consider a lifetime of uncapped downloads (with Premium Membership) at the value of £49.99. If you deduct the value of the 'extra' features associated with Supporter Status (at £1.29 for the life of the account), the cost would be £48.70. Not much of a difference, I think, by most measures.
I hope this clarifies what I meant a bit.
That said though, this does not address what you describe separately as the ability to "choose our own cap speed for a certain amount of money each month". That's perhaps something we can consider and I'll be happy to let the rest of the team know your thoughts about that. Thanks!