When does gold digging become prostitution?

I think when you say “that’s gonna cost you fifty”, you tip the scales towards prostitution.
Broadsheet – Salon.com

It’s a story as old as, well, the oldest profession. Beech skirts around that stigmatized word, using euphemisms like “sugar daddy” and “benefactor,” until acknowledging that outsiders might see her business arrangement as “the distant cousin of — dare I say it? — prostitution.” How distant is it, though? We — or rather, those who like to moralize about such things — make a classist distinction between high-class call girls and streetwalkers. The difference: Purchased sex that is more expensive, and purchased sex that is less expensive. Which is to say: What, exactly, is the significant difference? Is it suddenly not prostitution if a john takes an escort out to dinner before having sex with her? No, of course not. So, why would it not be prostitution if he establishes an ongoing business relationship with her, in which he pays for her sexual companionship?
As Hanna Rosin points out in XX Factor, this particular “sugar daddy” arrangement is not all that far from those of many self-described prostitutes: “The Internet has made it possible for prostitutes to fly solo and not get burned. They interview potential johns in public places, usually expensive restaurants. They ‘date’ for a long time — sometimes months — before agreeing to pair up. They may even hire a private detective to check the guy out.”