Your assumption that one person can serve 50 customers a month seems to be quite low. That comes to spending about four hours per customer to respond, track and provide a resolution or workaround! Of course, the duration would vary depending on what kind of support channel is used (phone vs. chat vs. email). You also seem to be assuming that all paying customers would have major issues that can only be resolved by working with other teams, getting a new resolution or change implemented, etc.

In reality, most customer cases can be resolved quickly by searching for documentation and pointing them to it and/or explaining things better (and improving the support documentation along side when necessary).

I would assume that at least 30 customers can be served per eight hour working day. That's spending more than 15 minutes per customer and is usually adequate (on average) even for phone support! With my extrapolation here, which is again just an example (real scenarios would be different), the basic cost incurred by the service provider for the salary alone comes to about $7 per user per month. Add for overheads and it would still be fine to charge $10 or $12 per month. If service tiers are used, then there could be multiple slabs, mechanisms for support and turnaround times - all these would bring down the actual cost further.