I went the same route as Logojoy with Shodan and it's worked out great. Even charging $5/ month was a non-starter for most individuals whereas a $49 one-time fee is more palatable. And despite it being a one-time fee the monthly revenue is very predictable so there isn't much uncertainty in terms of budget planning. I would highly recommend startups to consider one-time payments, especially if they're targeting individuals and not businesses.