To be very clear about it, I don't work on anything collections-related at Prosper. That said, I can provide some general background. From what I've seen elsewhere, contingency fees generally range between 20% and 40%, depending on the type of debt and other characteristics, like seasoning, loan size, income, presence of co-signer, and so on, not to mention the size of the overall portfolio in question.
Mechanically speaking, those fees are taken off the top - if an agency collects $100, the creditors receive a check for ~$70. As this is industry-standard, I assume that Prosper's arrangement is similar, in which case it never even saw the $30 that some are decrying as "stolen".
On the debt sale side... if anything, the price for charged-off paper varies more significantly than contingency fee rates. Again, I know nothing about Prosper's relationships, but few parties are likely to be paying good money for unsecured debts in 2010. Unemployment is high, real income is flat, and there's a ton of defaulted paper out there to choose from. If I were to guess, I'd expect to see prices today of about 1%-2% of outstanding principal on unsecured debt.
To get back to the larger questions, I've noticed two major themes in this thread. One is the idea that Prosper should cover collections expenses, but loan servicers just don't do that. That's just an economic reality imposed by the difficulty of collecting on the paper and the structure of contingency fee contracts. The alternative to a fee pass-through would be a debt sale at 1%-2% or no collections at all, and in most cases I would expect net life-of-loan collections to be better than that, even on an NPV basis.
The other theme is about the transparency of Prosper's collections efforts - like how much is taken from individual payments and why, as well as communication around what exactly is being done. I think that's potentially a more fruitful line of inquiry, albeit one that's easily drowned out.