This is a great post Danny. Thank you for raising the subject. It's a conversation I have with my peers often- what is the value of a Tweet? I don't know that there is a price that can be placed on a Twitter endorsement yet. Are you an individual, or are you a Corporation? Do you have a large and relevant following that will re:tweet your message? Are you including a link, or no?

Initially, I think the best way to handle this is to put some form of fee on Corporate accounts. Corporations (including brands and agencies) send endorsements that theoretically impact their ROI. If their endorsements have been effective thus far, it would make sense that a monthly/annual flat usage fee would be reasonable. For those who have not yet tested Twitter to see if it works, I think there are plenty of case studies that show the benefits.

As others have said, endorsements are often not suitable for individual accounts. This is why initially, I think there needs to be a distinction. Corporate accounts cost (a small) chunk of change. Individual accounts don't.

Perhaps a model such as this could even reduce the number of accounts wherein there is endorsement abuse; promotional link after link with little reader value?

There is clearly some monetary value for a promotional tweet. It will be interesting to see what Twitter decides that is.