The pricing would be reactive - as popular attractive people get more messages, the more it would cost to message them, growing exponentially in order to limit incoming messages to approx a few a day. Unpopular people who got few messages would have a price that would go down to zero. This would hopefully solve the problem of women getting hundreds of messages a day, and men getting neither any incoming messages nor any replies to their outgoing messages.
It shouldn't be much of a problem to get women on board, as they rarely send messages anyway and it would always be free just to put up a profile and receive messages. And for men it would hopefully be a fair an transparent way to have confidence that their outgoing messages would be received and read (and have fewer immediate competitors), rather than just lost in the noise.
I don't have time for it now, but it's always been something on my mind.
Edit: Thanks for your comments. This is interesting. Lots of hypotheses worth experimenting.