Could just filter out any DMs that isn’t in your follow list or your friends follow list. And if you insist on getting paid to receive dms from the rest, a separate service where people can zap you to send a DM could solve the rest.
Discussion
Yes your idea could work too but it seems like it would be over complicating things too much.
Its not really about making money off DMs, it's more of a tiny barrier to stop scammers, low quality DMs and bots. Its not much money, most people wouldn't care that much to have to pay to start a DM. It's pennies.