Umm it really varies. Dunno how much the cheapest is, but a good piano teacher I had last time was around $200+ for 4 lessons and I heard that it can go all the way up to $80 per lesson for the very sought-after teachers
It depends on ur teacher's qualifications and experience. My teacher last time charge 470 for theory and pratical both for grade 8, 1hr30mins-2hr. She is able to coach up till diploma degree.
I ever heard 250 for both too(not sure how long), so actually it varies and for you to see also if it's worth the money when u attend. Currently I'm with my teacher taking diploma