Although it sounds as though gliders might be cheaper, it's not starting out for the private.
You'll spend about the same for a glider private that you will for a private airplane - most likely.
It will be cheaper to do the glider commercial than the airplane commercial. In theory you can have a commercial glider certificate by 25hrs total with 100 landings. (15hrs and 80 landings more than the private requirement) And then from there you can time build with the glider commercial giving sightseeing tours from 25hrs until you get 150 hrs, then do your 100 hrs in airplanes for a commercial SEL add-on.
the private glider (from the school where I learned to fly) is approx $6500. About 12-15hrs and 40-55 flights.
Commercial 10-13 hours and 45-60 landings more @ $30-45/flight hour and $30/hookup-tow to pattern altitude/$50 for a tow to 1500ft/$75 for a tow to 3000ft. You will most likely exceed the flight time requirement by the virtue of trying to get the landings (unless all you do are pattern tows) and getting out there practicing your soaring techniques.
Then like I said afterwards after you get the commercial. You're not paying anything for flight time to 150hrs as your pax are paying for your flight time. But you're probably not getting paid either, your compensation will be based on how well you bond with your pax and getting tipped for a good sightseeing flight.