Cost of hiring someone freelance depends on their experience, how much custom assets you want (sound, texture, models, etc) and how soon you want it. Typically though these kinds of things aren't going to attract any mappers attention short of around $200.

Contributing is different and depends on what you want them to do.

When you present your request be sure you are as detailed as possible in what you want done. I would suggest asking at the facepunch forums mapping section to begin with. Plenty of freelance guys and gals over there.