Yeah, the big problem is labor. if you are only making a few bucks a user, making enough money to make up for your opportunity cost (vs. working as a tech for a large company) requires a lot of users.

I've got a lot of experience serving the low-margin market, both for ISPs, WISPs, but mostly on the hosting side, (and hosting is actually easier; the less you have the customer on the equipment, the less time you spend fixing stuff they broke.) and let me tell you, low margin does not at all line up with low support expectations.

Even if you can charge your customer twice what it's costing you and making ten bucks a month per user, one tech support issue can end up eating all the margin that customer would have given you for years.