I'd prefer to pay per project rather than per month. I think this makes more sense at the lower end / for the "freelance" tier.
Say I do 5 web projects a year. Each one is reasonable budget, and mapping is a core part. From a psychology pov I'd much rather pay (say) $200 one off for each project (maybe based on a cost per page) fee than a rolling per month cost on a thing I'm only using intermittently.
Money wise, my response is silly, I know, but I think the psychology at play here is important. As a small company I don't like to spend money monthly on a thing I don't use whereas when client has just paid me a big deposit for a new gig, I'm happy to pay.
For larger tiers, cool. But a per page "once off" would make sense to me.