Actually, the first rabies is for one year. Usually if you get it boostered on time each vaccine thereafter can be for 3 years (I think most vets use the 3-year vaccine).
Keep in mind that the reason that vets charge more is that they're doing an examination on your animal and not just popping a shot. It's very important to keep shots updated, but at least yearly exams are very important as well.
I would expect a cat with rabies to be showing other neurologic signs and not just foaming. Most likely the cat got worked up or got something irritating in it's mouth.
