Depends on the contracts with the various recording companies and radio stations, but usually very little.
My friend is quite a well-known songwriter and she's just made a tidy little sum selling some of her songs (not her recordings) to one of the big recording companies. Not enough to retire on, but more than she'd get in straight up royalties.
Above is true. Also depends on the station, time of day, etc. A daytime play on Radio 1 can earn quite a bit �30-�90, but a night time play on Radio Wales only a few pennies. Trust me to be in a band that only gets paid on Radio Wales!
Even if a record isn't a hit, an artist can make quite a few bob from having a 'radio hit'. Again, not enough to retire on but makes life a little easier.