$119 - Flights to Las Vegas from San Francisco
If you can fly out on Thursday morning or Friday morning, and fly back to San Francisco on Sunday morning, the cheapest flights are with Virgin America. They offer direct flights for $119 including all taxes and fees.
For Friday night flights, Virgin America offers flights for $179 (flying back early on Sunday), and United is offering flights for $177.
If you want to book a last minute flight to Las Vegas, US Airways is the cheapest, as flight with Virgin America especially tend to fill up.