Has a service been exchanged for money? If the answer is yes then I think that's a clear enough way to distinguish it.
Let's say you have a friend who runs a coffee shop, for example. If you pay for a coffee, that's a business transaction. If they offer you a coffee for free because they like you, that's not.
The end result is the same, you got a coffee, but the dynamic is entirely different.
Once the client becomes the boyfriend, he no longer pays for sex and the dynamic is completely different.
I do agree that wealthier people have more options, as I agree that poor people are way more likely to resort to prostitution. I don't think prostitution is that ambiguous though.