Wow. That was fast.
MGM Resorts in Las Vegas started charging for parking in  June of last year. They are changing the fee structure and raising the rates as of April 12, 2017.

The new Self Parking rates are:
Excalibur, Luxor & Monte Carlo:

• 1st hour is free
•  1-2 hours is $5
•  2-4 hours is $8
• 4-24 hours is $10

MGM Grand, Mandalay Bay, Mirage and New York – New York :

• 1st hour is free
•  1-2 hours is $7
•  2-4 hours is $10
• 4-24 hours is $12

Aria, Bellagio, Vdara :

• 1st hour is free
•  1-2 hours is $7
•  2-4 hours is $12
• 4-24 hours is $15

Note: Self parking is still free at Circus Circus for now.

The new Valet  Parking rates are:

Circus Circus,  Excalibur, Luxor & Monte Carlo:

• up to 4 hours is $10
• 4-24 hours is $15

MGM Grand, Mandalay Bay, Mirage and New York – New York :

• up to 4 hours is $15
• 4-24 hours is $20

Aria, Bellagio, Vdara :

• up to 4 hours is $20
• 4-24 hours is $25

M life Rewards Members

• Self-parking is still complimentary for Pearl, Gold, Platinum and NOIR M life Rewards Members
• Valet-parking is complimentary for Gold, Platinum and NOIR M life Rewards Members
• Scan your qualifying M life card for entry and exit (you do not need to take a ticket)