It does in fact have to carry a few miles. The closing speed of a 150kt spamcan plus a 737 can be quite high. A few miles' range needs more RF power than one is going to get with small lithium cells.
Anyway, as has been written here already, 1090ES is the way to do this. It's established and internationally accepted.
The reason prices are not as low as they could be is that certified avionics is an international cartel, allowing stuff to retail at 10x to 20x manufacturing cost. Everybody except the end user loves it; the avionics shops like the reseller margins, the installation charges, the distributors like their margins... and this "find a stream of money and position yourself alongside it" is what keeps the whole world rolling along nicely
On Permit planes, one could do a lot.
I am sure products are being developed right now. But nobody, not even a bold new player who has nothing to lose in bombing the market and pi**ing off the distribution networks, is going to announce a product now, before Mode S is mandatory. This is because he will know that Garmin, etc, could push out a sub-£1000 version of a GTX330 within a few months if they wanted to, and why show your hand early?
BTW, a KRA-10 is a
radar altimeter
https://www3.bendixking.com/static/c...ils.jsp?pid=83