Much calmer now ... Whooshaaaaah ...
Interesting calculation about the amount of area that a 20nm radius covers though.
The times I had occasion to hear 20 mile calls from the RPT were going into Longreach, where we were about to arrive at the same time as the flydoc. He was 20 miles out and we were about 5 miles out. It was nice to know where he was and in the end, he beat us to the airfield.
The other time was at Ayers Rock, there was a 737 who started calling the CTAF about 50 miles out, from memory. Those things turn base about 10 miles out; Scary!
I'm not trying to tell you what to do, but I think a transponder would be better value for your money. Here's my reasons
1) You already have a radio, a transponder would add new traffic awareness value to your plane. A dual watch radio would only enhance what you already have.
2) RPT traffic have ADS-B receivers on board. Whether you're on the right radio frequency or not, they will see you, even if you're not talking to them or even aware that they're there.
3) In high traffic areas such as SE Qld, I have had ATC call me a number of times with traffic alerts, because they could also see me. Usually, they ask me to squawk ident to check if it is me, and every time it has been me, so the transponder may have already saved my life, my wife's life and someone else's, several times.
4) Around the coast, a transponder is also useful as an emergency beacon, by squawking the emergency code. ATC can track your position and direction of travel if you get it on early enough and at enough height. Every little bit helps.