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This is why it is nearly impossible to study protitution scientifically: How does one define it? If something is real it must be definable and measurable.
These types of "arrangement" are so common as to be uncountable. They are extremely common, for example, among affluent Asian businessmen and often with the tacit approval of the spouse in a don't ask, don't tell arrangement.
The question here -- is gold digging prostitution -- is as old as dirt. One reason there is no "good" answer is that nothing will ever change so long as attractive -- and younger -- women have something to offer in the boudoir and men have money and power.