Depending on the stallion, horse semen is one of the most expensive liquids on the planet. A gallon of gold-medal-winning Big Star's semen is worth $4.7 million. But that's nowhere near the most expensive! Once it's collected, horse semen can be sold in small tubes called straws, each containing 150 to 200 million sperm cells. These straws can sell for thousands of dollars. In the horse-breeding world, genetics is king. Wealthy investors are willing to pay high prices for a proven winner's semen, hoping that the resulting foal provides a large return on investment.