I tried an internet searches for "royalty earnings of..." both singers in question and, perhaps understandably, neither threw up any figures and only in the case of Holly were there any significant hits. There were a number of sites detailing how the inexperienced young Texan had been ripped of by the big record labels. The stories are horrific if not unexpected.
A quote from one said, Holly only received 16 2/3s percent of the songwriter royalties from The Crickets first hit, “That’ll Be the Day.” when it was well known that he wrote both the lyrics and the melody.