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It depends on the the service. I use freshbooks, and their plans are roughly based on number of customers you have. I don't mind paying more the more customers I have, because it means I'm making more money. I don't mind spending money when I'm making money.

One thought is that the bigger your customer is, the more they can afford to pay for the service. You don't want to leave to much money on the table. Its pretty normal for people to undercharge for their services. Do you have any feel for if you are going to get more 1-3, 4-10, 11-25 users?

The things I don't like about tiered services, is that there is a big price jump when you move between tiers. Moving from 3 to 4 users in your example doubles the cost to the service. I'd make more gradual tiers rather than big jumps.