The simple answer to Sholva's question is that the lowest-cost way to listen to ATC is to buy an airband receiver. There are many on offer: anything continuously tuned will be awkward to use unless it has a digital frequency display. Digital synthesised receivers are available at (relatively) reasonable prices.
Regarding listening to ATC: in some countries (such as the UK) it is prohibited unless you have a licence, and even then only when in an aircraft. In other countries there is no such regulation, and anyone may listen.
I have never heard of anyone being prosecuted in the UK for listening to ATC (or indeed to much else), but that doesn't change the technical legal position, to which I defer to FL.
One of my former licences said I was not allowed to listen to any frequencies other than those covered by that licence. If I should accidentally hear anything on those frequencies, I was not to record it. If I should accidentally make a recording, I was not to play it to anyone not authorised to hear it. A very strange set of rules!
What is an absolute no-no is transmitting on airband other than for genuine aviation purposes.