I feel as if this is a tough question without some sort of context as to what kind of dining you're planning on / how many souvenirs / what type you're interested in. It's just that the range is huge looking at the different possibilities for what you could spend money on.
That said, I think $75-100 as previously suggested would be a nice, comfortable amount if, say, you were interested in having two decent meals throughout the day and a few snacks between, coupled with some basic souvenirs from here to there and a couple of nicer things.
I think you could probably pull off $30-40 a day if you were really working to go on the cheap, and I'm sure it's not all that difficult to spend several hundred a day if you're enjoying the nicer restaurants there and really spending on the different gifts (for yourself and others) that you can buy (there are some high end products for sale...).