If I was making a FPS game then I'd want all the levels to be done by the same person, or team even - otherwise there would be no continuity. In that case, I'd probably expect to pay about £1500-£2000 for 10 levels (maybe 2-3 days spent on each level) - depending on funding. It is conceivable that a publisher would pay for good levels to be created, I doubt anyone has that sort of money just waiting to be spent on a hobby
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Or, I'd get some specialist models made and make levels like they do in the Battlefield games - like get a load of different models, buildings, busted vehicles, furniture, building blocks that can be easily re-used in a simple level editor.
Making big multi-purpose models might be a good idea if your looking for stuff to make and sell. Even if you got asked for a specific thing, having a similar model in your library would be pretty handy - say you make a hospital building, someone could then request a survival horror theme to it, then it would be a case of changing it to suit, or as is usually the case, splatter buckets of blood over the walls and floors, as if dying people have nothing better to do than bleed and wipe it all over the place.
If I was in the media creation game, then I'd probably do that, have a library of cheaper media that people can buy, then charge more for specific changes or original work.
