I'm 19, and I do sculpting for Alternative Armies.
I don't expect money at first, I expect to show them I care and that I am well organised and keep to schedules. When I feel ready, I might mention the idea of working based on a contract.
As for how much, I don't know. It depends on the quality of work, how long it took and how well it will sell.
I couldn't speak for the quality, but if it's quite generic, then it should be quite popular.