The only way to understand the Court's Commerce Clause jurisprudence is by turning to chaos theory. Chaos theory tells us that if a butterfly flaps its wings in Hong Kong, it may cause a hurricane in Texas. "Fundamental to chaos theory is the phenomenon of sensitive dependence on initial conditions, commonly referred to as the Butterfly Effect." Leonardo Electronics Almanac.
If I sneeze in California, it may cause an earthquake in Missouri. Hence, Congress has the power to criminalize my intrastate sneezing because it may substantially affect interstate commerce. (After all, an earthquake can cause billions of dollars in damage. Everyone has heard of the million dollar man. But had you heard of the billion dollar sneeze?
Even one dollar spent in Utah will have a substantial affect on interstate commerce since this dollar will travel across the country many times. This website explains why.
Hence, the Commerce Clause confers upon Congress to regulate any activity it likes, so long as it does not offend the Court in so doing.