This simple little FM transmitter from eBay for the incredibly low price of $1.69 is more powerful than one would expect. Range is up to 500m with a 9V battery and a quarter wavelength antenna wire.
eBay links are always changing, search for "FM transmitter module wireless microphone", sort for "buy now" and "lowest price" to find it.
The frequency is adjustable and pretty stable due to an additional amplifier section. Audio quality is very good!
Only very basic soldering skills are needed to get this up and running!
