I have a friend who graduated OCS within the last year, and he indicated that it is a very wise idea to save money ahead of time. The largest expenses were uniforms (in the $2500 neighborhood), and going out during liberty. He thought that there would be little to nothing to spend money on while at OCS (other than the recommended cash). In addition, if you are currently receiving pay from the military, you will lose BAS & BAH, and it can take another 3 months to get it back (but you will get backpay).
Moral of the story ~ save some cash before you head to OCS and you'll have one less thing to worry about while you're there.
Close...
Here's the straight dope on pay at OCS.
Unless you're in a college program that pays you at a rate higher than E-5, you'll get E-5 pay. If you are in a college program or are prior service, then you'll also have been getting BAS and BAH in addition to your pay. The big difference when you get to OCS is between people with dependents and people without. For the people with dependents, there will be little change. You'll lose BAS (about $280) because the gov't is feeding you, but you'll get a family separation allowance (about $250), so it more or less evens out. You'll continue to receive BAH.
What bit people in the hindparts, though, was that the folks that didn't have dependents lost both BAS (and didn't get FSA) and BAH. That's a large chunk of untaxable change. Furthermore, the paperwork didn't make it through PSD until about Week Six or so, so the first month that we were there those folks continued to receive their allowances. And as the government giveth, the government taketh away--so some folks got $2.45 pay deposits during one cycle, and found their (now fixed) pay on the following cycle to be greatly reduced from what it had been. You will NOT be reimbursed this money at a later date, but once you get out of OCS you'll receive your allowances again. So I would agree with nugget and strongly advise those without dependents going to OCS to make sure you have a bit of a cushion.
As far as what you'll spend...You'll need to have around $1000-1200 of your own money for uniforms. You can borrow that, if you need to, from the NEX. They give you a no-interest 12-payment chance to pay it off. I left with a bill right at about $1000, and I pay around $83 a month. You can also put that on a Star Card, as mentioned above. $300 is more than enough to bring with you, and you'll be able to go to the NEX and use the ATM there fairly soon after arrival if you need to. How much you spend on liberty is up to you.