I certainly have no plans on living like a king and expect to have a roommate to share rental expense - I thought anyone here would know that but apparently not...
The $1,800 amount is before any taxes. One should probably figure around 11-13% for income taxes (with the first year being less assuming you don't have a full years income.)
Also in California you will pay another 7.5% off the top that is required by the state for your savings. (This money is a savings plan and can later be rolled into an IRA after you leave your program but will not be available for your living expenses)
If this link will work below you will see that this appears to be an issue that has developed in the last few years and that California is becoming known for not being competitive with stipends.
http://senate.universityofcalifornia.edu/CAGSSGradCompetitivenessPaper_072012.pdf
(If the link doesn't work google: task force on competitiveness graduate support )
You may also want to be alert to the wording in your offer letters. Other offers have mentioned that the stipend amounts are expected to increase in future years whereas the UC is silent in this regards.
Also of interest from the report above is that TA pay is the same system wide based on a collective bargaining agreement whereas the RA pay is determined at the local level. This may explain why TA and RA pay is different at the UC's with RA often being lower.
All of this isn't to say to go to a UC, but go informed with this information and your eyes open, as the monthly difference over 48+ months starts to add up and it is disappointing to see that this is known yet remains unaddressed, perhaps hoping the students really won't mind. (They can always take on debt or get bunked beds as needed right?)