I'd be delighted if it did turn out to be cheaper -- I'm a great fan of ADS-B.
Would you have thought sat phones, or even mobile phones, would be so cheap so short a time later, when the first few people got them?
Sure. So now we have billions of mobile phones in the world, and they cost almost nothing. Meanwhile the good old VHF AM transceiver, a vastly simpler technology, still runs to 4-digit prices if you want a class 1 (approved) unit. TSO-129A GPSs have been around for 10 years, and the price really hasn't fallen much. The overall market is just not big enough to bring the prices down.
It may be that the "lets-go-hiking" accuracy of +/- 50m is quite accurate enough for the purpose contemplated.
It's not the accuracy, but the certification of reliability that is the costly issue. The costs of certification have to be recovered through the price of the units sold to the aviation market.