We stay at this hotel frequently. There are really only two options:
Sutter-Stockton Garage across the street; $34 for 24 hours or hourly rates.
Union Square Garage one block away ; $32 for 24 hours or hourly rates.
Here's a link to the rates:
http://www.unionsquareshop.com/parking.html
The website for this hotel shows its parking rate is $49 PLUS TAX.