Complex sentence: The economics of coal revolved around two factors: most of the cost of production was wages for miners, and if the supply fell the price would shoot up because in an age before oil and electricity, there were no good substitutes.
Split into simpler sentences: The economics of coal revolved around two factors: most of the cost of production was wages for miners, and if the supply fell, the price would shoot up. In an age before the use of oil and electricity, there were no good substitutes.