Greyhound
Running at least five buses daily from Phoenix to Las Vegas, Greyhound is the only consistent option when looking for a passenger bus direct to Las Vegas. Prices can range from $40 to $60 and discounts are offered to veterans. Greyhound also offers companion fares. Tickets can be purchased online or at the Greyhound Station on Buckeye Road in Phoenix. Typically, Greyhound buses are the less comfortable option, but are more reliable --- Greyhound has been in operation since 1914. Many smaller providers have floundered, like Megabus, which discontinued its West Coast operations in 2008 because of poor sales.
Chinatown Buses
Though more prominent on the East Coast, Chinatown buses have been slowly making their way west. Often, these buses have unconventional, storefront pickups, but the price is unbeatable. There are no direct routes from Phoenix to Las Vegas currently, but for sake of adventure you can easily catch one of these buses to Los Angeles for $30 and head on to Las Vegas for only $15. Look out for direct routes in the future, as the bus operator, GotoBus.com, has been expanding in the West.
Charter Buses
There are a wealth of available buses for charter in the greater Phoenix area and options can be found online. Prices for charter buses vary greatly, so you will want to check out a few providers. Remember, when you charter a bus, you are buying the whole thing, so this option is great for large groups, but individuals or small parties may want to consider Greyhound or Chinatown buses. However, if you want to stop and enjoy the scenery and sites along the way to Las Vegas, and you have enough people or funding to choose a charter bus, you may find it worth your while. Check out Busrates.com to find a comprehensive guide to charter buses in your area.