One on one instruction is, by its very nature, fairly expensive. I may sound like I'm repeating what others have said, but I really wish people understood that "$xx/hour" lessons does not mean we are earning $xx/hour. Not even close!! Piano teachers earn far less per hour than other professionals, and most of us began our training early in childhood, spending countless hours (and dollars) on our own private lessons, not to mention college education.
I know that for every hour I spend teaching each week, I spend at least an additional hour in preparation (ordering new music, typing newsletters, doing bookkeeping, designing customized worksheets to meet students' specific needs, planning recitals, etc etc etc). So instantly my salary becomes not $xx/hour, but rather half of that. Then of course I have many expenses (frequent piano tunings, professional memberships, music books, teaching aids and games, recital expenses, teaching textbooks and magazines, business insurance, private health insurance, self-employment taxes, on and on and on!) For me, those expenses take nearly another half off my hourly wage. When all is said and done, my "hourly wage" ends up being about 1/4 of what a students pays for an hour of lessons.

The only way you can expect to pay much less is if you start out in a group class. Most children receive their academic education in large classrooms, because few could afford a private tutor for their studies. Similarly, if you are willing to learn piano in a group setting, the cost may be more palatable for you. Check around to see if any adult group classes are available in your area.