I'm assuming Starbuck isn't much different in price, and that may be the case. If that is the case then its probably because Starbucks is selling overpriced coffee that costs pennies... Whereas a McDonalds is selling food with a very small margin which is why it is so cheap. If Disney is charging a very high rental for the space the Starbucks can easily afford that extra rent as it is like simply increasing the cost of the coffee it sell by a dollar or so... not a problem if you were making 3 or more dollars margin before... but if you were McDonalds that was making very little on some items you would be selling at a lose and couldn't stay in business without raising your prices.
It is also possible that Starbucks may have been given a special rate by Disney to get them into the park because they thought it would be a draw to visitors... While they might do the same with McDonalds, why would they do that when the McDonalds would be competing against their own burger and fries in Tomorrow Land? I don't think they would give McDonalds any price breaks so you would see your prices go up.
