Complex sentence: By 1930, because enforcement diminished the supply, this had increased to $35 per year (there was no inflation in this period), resulting in an illegal alcohol beverage industry that made an average of $3 billion per year in illegal untaxed income.
Split into simpler sentences: By 1930, because enforcement diminished the supply, spending had increased to $35 per year (there was no inflation in this period). The result was an illegal alcohol beverage industry that made an average of $3 billion per year in illegal untaxed income.