Mweh. Too simplistic of an approach in my humble opinion.
First of all it's technically challenging to charge for a reply/DM. The amount of negotiation to get a DM/reply through will kill the user experience.
Secondly I don't think this is a guarantee for no spam/nasty stuff. Over time you'll find yourself looking for relays with a higher admission fee than 1 sat because some spammer has figured it out.
Why are you reading DM's from people you don't follow?
Just use a WoT relay and you don't need to think anymore.
I'm just using common relays and I hardly see any spam.