Roger, this becomes very clear with free to paid accounts upgrade example.
Take a look: GitHub, Sumo Logic, Codeship, Dropbox, etc. — they offer you free accounts while you are small, probably not making money, have ‘small amounts of work to do by them’. But when you cross the bar, service fees are going to be applied — because you’re making some profit using their service and they are going to cut some percentage of that profit (maybe non-financial profit though).
To make that percentage ‘fixed’ they develop different service plans for ‘different amounts of work to be done’ (5 users plan / 50 users plan / 500 users plan, etc.) — this way they try to adjust their prices with the client’s growth (i.e. client’s profit growth) — and percentage of profit becomes ‘so-so fixed’ for a client.