A study tries to figure it out, based on Obama campaign contribution info:

Political ambassadors who had made campaign donations of $550,000, or bundled contributions of $750,000, had a 90 percent chance of being posted to a country in Western Europe... Luxembourg came in at the top of the chart, with a posting there valued at $3.1 million in direct contributions, while an appointment to Portugal was predicted to have a value of $602,686 in personal contributions.