If there is such a big difference in what relenders donate and first timers donate, it sounds like Kiva needs to get the point across that operating costs are not tied to the amount of money you have in the lending system. It is more tied to the total amount you have lent, since they have to keep up the engineering, accounting, oversight, and troubleshooting for all current loans, irregardless of whether the money was 'new' to the system or not. Also Kiva is trying to reach more borrowers and partner with new MFIs to meet demand, which is hardly correlated at all to the amount lent at the moment.
Maybe on the donation screen they can emphasize that they are trying to keep up with demand and need additional funds not only to maintain the loan the person is making at that moment, but also to try and expand into new markets. I am not good with words, but somehow they need to make that point.
It seems obvious to me that the amount Kiva needs is not really that tied the current amount loaned at all, which is part of the problem for both Kiva - trying to get enough donations - and lenders, who are unsure what is really needed. The hard part is communicating and convincing the potential donor of the real need, yet still making it simple and ultraconvenient for everyone to contribute.
I am not so sure about membership or any such thing, as it seems it would complicate matters, and also lead to more engineering requirements that the membership fees alone may not be able to support. I guess that is up to Kiva to figure out.
I for one would like to see some more guidance from Kiva as to what they need and how that would calculate down to a % of the current amount lent. Personally, this seems like not a stagnant number, as Kivas status quo vs different levels of expansion all cost different amounts, and the amount lent is constantly changing. But I also understand that not all people give and so that is why charities ask for more than each persons share would be. So I don't know exactly how Kiva should go about it. I kind of wish they would actually tell those of us who are more involved some real numbers, so we know what is a reasonable amount to donate, even if they leave the psychology based figures on the website. But of course that I guess could defeat the purpose of the psychology based figures.

But I do feel they need to spend a little time rewording and 're-marketing' their donation approach to make it a little more obvious that donations should keep coming even if you aren't adding more money to the system. I think their first effort should be to put together a page that people could link to from both the ABOUT section, as well as from the contribution request page that describes in some detail what the donations support: Kiva fellow training, staff in 5 regions of the world to train and oversee MFIs, legal and administration costs of getting new MFIs on board, engineering support for both the lender face of the website and the MFI interface, etc. Not a huge deal, just a short breakdown of some expenses so people get the idea that the donations aren't just needed the first time money is put into the system. This somehow is a thought trap that is easy to fall into with Kiva, as I know I went through those thoughts before too. I remember wondering if I should continue donating on relent money, and if so, should I donate the same % as I donated on the original loans? Maybe on the need page they could actually breakdown their need types into three categories 1) any costs associated with depositing or withdrawing money, or setting up a new account - anything that is a one time expense 2) expenses associated with upkeep of all current loans, MFIs, and systems 3) expenses associated with getting new MFI partners, expanding into new markets, getting new lenders, offering new services, etc, etc
OK, I will quit now
