If you have no idea what you should demand for your levelwork,
just do a simple calculation:

Worktime per level * medium salary for a designer.

So if a level takes you 20 hours and you take a 15$ salary
it would be 300$ per level.

this is just a speculation, but on a competaitive marked, the prices will
level out on the average production costs. (regarding that others
will offer level-work too to the company)

in the end the price is allways what both parties are willing to agree on.