Complex sentence: Later sales were steady and would have given Joplin an income which would have covered his expenses; in 1909 estimated sales would have given him an income of $600 annually (approximately $in current prices).
Split into simpler sentences: Later sales were steady, and would have given Joplin an income that would have covered his expenses. In 1909, estimated sales would have given him an income of $600 annually (approximately $in current prices).