I think as long as the party receiving the DMs has it as an option it is fine.

i'd prefer the following if it was being done just for me....

1. Unknown parties must pay me 100 sats to DM me

2. Once I follow you back you pay me nothing to DM me

If it is to "prevent spam" vs. a way to be enriched as an influencer at the expense of followers, let's treat it like that.