I think that is a good rule of thumb. Maybe 10 percent of pre-tax income, especially as you get older.
Religious tithing has long been 10 percent of pre-tax income, although most countries give a tax deduction for religious donations.
Church leaders say that ten percent isn’t much. Let’s assume they are right about that. Many Johns aren’t religious, so substituting commercial sex for religious tithing seems reasonable.
I think there are arguments for the percentage to go higher as a person gets much older. While I have long argued that there is absolutely nothing wrong with younger guys purchasing sex, it makes sense to combine dating & commercial sex into a category that is a relatively high percentage of total income. It allows the guy to explore what he likes best, although it is necessary to develop compartmentalization skills to not mention this to civilians he is dating. Some guys get their greatest enjoyment in life from kids and the (often brief) times that the civilian relationship goes well.
As a guy gets older, there is often less enjoyment from dating if most of the dating prospects are old Gertrude, who makes casseroles. Some on the board actually advocate never participating in this activity because it will discourage your attractiveness to old women as you get much older. That is a pretty depressing way to look at life. Like never eat chocolate or steak so you will always think that vegetables are tasty. So perhaps that larger Dating and Purchasing Sex budget item gets more heavily weighted to Purchasing Sex as you get much older.
On that same vein, some much older guys are actually transferring wealth to younger generations by purchasing sex, and god knows, in the U.S. at least, we need better wealth redistribution. A guy with no children, much older grown children, and/or no grandchildren, might very reasonably spend more or his current income and accumulated wealth on sex workers, in comparison to the guy with fewer descendants, or descendants who spend any time with him or extend any emotional connection to him.
Just my thoughts. Ultimately the percentage of income/wealth spent on this that generates maximum happiness likely varies from person to person, but I think the one you threw out is pretty reasonable. As I said, I could argue a higher percentage, especially as a guy gets much older.