Where I live, Montessori ends at Kindergarten, unfortunately. I have three kids in Montessori pre-K right now, and it costs more than the mortgage on my house. But as other commenters have said, it's worth every penny. Kids are calm, happy, and engaged. They love school and learning. I'm not looking forward to public primary school, which is our only realistic option.
Yes, this is true in a number of areas. Even other Montessori schools in our area usually stop at Kindergarten. We're extremely lucky the one our son goes to runs one or two classrooms (based on demand) up through 5th grade.
There are some very good private schools available after that, but those are 2x-3x the annual cost we currently pay. Luckily, we have a few years until we have to make any decisions.
That said, I agree with you: It's worth every penny. I would work nights at a fast food joint, scrubbing the bathroom floors with my own toothbrush to keep my son there. He loves it more than we do.