Dorms are free, they will pay heat, water, elec...you will have to pay for phone, cable, internet, etc.
Base pay for an 0-1 is somewhere around $2300/month. Flight pay is $125 once you start UPT. BAS is around $175/month. Take out normal taxes, $20 for SGLI (life insurance) and I'm sure there is more, but I can't remember right now.
How much you spend is up to you...eating out every night or the Super Walmart?
ROTH IRA and TSP are good investment tools for you.