From the SEC vs. Elon Musk filing:

According to Musk, he calculated the $420 price per share based on a 20% premium over that day’s closing share price because he thought 20% was a “standard premium” in going-private transactions. This calculation resulted in a price of $419, and Musk stated that he rounded the price up to $420 because he had recently learned about the number’s significance in marijuana culture and thought his girlfriend “would find it funny, which admittedly is not a great reason to pick a price.”

https://www.sec.gov/litigation/complaints/2018/comp-pr2018-219.pdf