Swim Like a Fish has group lessons - no more than 4 in a lesson - for $20 per lesson. Anyone younger than 2 has to have a parent in the water with them. My kids love it there.
I've heard Swim America is really good. They are through the local high schools. I don't know the price, but the benefit of SA is once your child achieves the goals for that level, they move right up to the next. They don't have to wait out a specific number of lessons.