From what I've read I don't believe there are any patents covering the basic concepts. Additionally patents expire after 20 years, and there is definitely prior art covering the basic device going back at least to the 60s.

There may be patents covering some specific implementations, but I believe one could design a patent free device fairly easily.

Actually, the real cost would be getting the necessary FDA certifications and/or funding the lawyers that defend your right not to need a cert.