Can you find a cheap one way flight from PHX to LAS? Perhaps Southwest? If so, make the car reservation to return the vehicle to PHX.

However, before doing anything check whether the Grand Canyon is closed due to the shutdown of the federal government. If so, do not assume that it will immediately re-open when the government does; there could be a delay of as long as several weeks to allow government employees to make the area ready to receive visitors again.

BTW, during the last such situation, the Arizona government apparently paid to keep the South Rim open for tourists, but I haven't heard of that happening this time.