Design and advertising agencies are often asked by their clients to produce apps, and generally have a single inhouse dev or outsource the work to freelancers.

Do some research in your area to find a fair hourly rate for your work. Bill hourly, never ever agree to a per-project fee, I cannot stress that enough.

Also I would recommend against accepting any offers of full or part-time employment with the agencies. They tend to be sweatshops. Treat them as a supplementary source of income and don't get sucked in too far.