...substantially less than half.

Of course, that depends on your definition of escort.  

At one extreme, some would say that marriage is a form of escorting inasmuch as there is an implicit financial component to a relationship that involves sex.

At the other extreme, an escort is only an escort if they guarantee sex to strangers during a fixed amount of time for a fixed amount of money.

Between these two extremes there is a lot of grey zone.

If you buy a girl dinner and chocolates and flowers and she sleeps with you does that make her an escort?

What if you buy her jewelry?  What if the jewelry is worth 10s of 1,000s of dollars?

What if you pay for the rent on her apartment in exchange for the privilege of dropping by and banging her when the mood strikes you?

What if instead of paying her rent, you just give her $3,000/mo and she's on call to come by and shag you at your place when you want?

What if you both find that it makes more sense to give her an envelope of cash each time you see her instead of a fixed amount each month?

Really the question is about where you draw the line between an escort and a friend-with-benefits--and that's a subjective matter.

I think there's a really big niche out there for girls who are not comfortable with the idea of fucking a lot of different strangers for compensation but are still comfortable with doing one guy for compensation.  These girls are more likely to turn to a sugar daddy site as opposed to CityVibe.

Also, there are a lot of girls out there who just want to date rich dudes and aren't looking explicitly for cash compensation.  For them it is enough to be driven around in fancy cars, go to fabulous restaurants, stay at 5* hotels and travel on exotic holidays.  These girls too are drawn to sugar daddy sites.