There's a number of legal fees which you could incur, however if you are willing to put in the work you can do you best to mitigate the total expense. (1) You'll have file with the Secretary of State your Articles of Organization, if you're choosing to go with an LLC, just know there are other types of companies out there such as, Inc.'s, S-Corps, etc. You'll probably want to talk to your accountant to find which is the best one which suits your purposes. (2) File with the IRS to get an EIN or Employer Identification Number. (3) File with all the state/local agencies to whom you will be sending your taxes. (4) I'm not sure if you're going to have employees, but then you'd have to file additional information for worker's comp insurance, etc.
That's about it to get up and running. The costs associated with the filing vary from state-to-state, as well as, the regulations that are required when setting up. Some states are more restrictive in that they can charge exorbitant fees to get your business tax license and thus require additional capital to ensure you can get up and running. Most of this information will be on the Secretary of State's website and then look under your applicable Chamber of Commerce which will have additional information.