The Tinkerers are able to thanks to the millions of Raspberry Pi and RTL-SDR units sold. If they used hardware which were only sold to US customers and only compatible with a small group of users, they it would cost a lot more.
But it would be nice if it were a little cheaper. You already get traffic from the transponder, so you're gaining weather with the SV-ADSB-470, which is great, but not a deal breaker.