See the below. 18K on top of tuition and insurance is not a bad offer, but it's also not very much in the DC area. That's only $1500 a month (assuming you spread it over the year), assuming no taxes at taken out (and Virginia has a state income tax, on top of both federal income and FICA taxes, assuming they withhold). It's possible to live on this, but you'll have to get roommates, no other way. For example, the cheapest studio apartments in Northern Virginia go for upwards of $1000/month. Good luck.
From here: http://provost.gmu.edu/support/presidentialscholar.html
The Presidential Scholarship
What is this scholarship?
The Presidential Scholarship is a program of support for first- time Ph.D. students. It is intended to assist George Mason in recruiting doctoral students of the highest caliber, and is offered and administered by the Office of the Provost.
Who can receive this scholarship?
This scholarship is offered to students by the Office of the Provost. Students are nominated for this scholarship by the department they have applied to.
Requirements for nomination include that the student:
Be a first-time Ph.D. student, and
Have a minimum GPA of 3.5 in the most recently earned degree, and
Have a minimum combined math and verbal GRE score of 1,200 (score earned within the last five years).
What is provided by this scholarship?
The Presidential Scholarship includes:
A Graduate Research Assistantship (GRA) from August 25 through May 24, and
A salary of $18,000 from August 25 through May 24, and
A tuition grant covering nine credit hours in the fall and nine credit hours in the spring, and
Subsidized health insurance (provided that eligibility is met).
The maximum term of the scholarship is three years (six semesters). Students cannot receive more than one term of this scholarship.
Scholarship renewal is dependent upon the scholar having a minimum cumulative 3.5 GPA at the end of their first and second years under this scholarship. They also must be making continuous progress towards their dissertation, as determined by their graduate program.