Interesting Question! :)
I for one don't really consider "gold diggers" as prostitutes. The main difference between the two is how the other party benefited for the act.
Prostitution is mostly business. Prostitution pays the other party something (usually money) in exchange for the services rendered. Gold-Digging on the other hand do not receive any form of payment. They trick and seduce the other party (usually rich men/women) into giving them something.
So Prostitution pays the other party for the services rendered while Gold-Digging gives the other party something because he/she was tricked or deceived into giving.
In my opinion gold diggers are selling also something which is their bodies and time. I don't think that they would only go for a dinner with a "hooked" ugly but rich guy and then each to their home, so accepting to have sexual relations with someone even if not for just one night in exchange of other benefits it is prostitution. That's why I also asked it prostitution is a state of mind.