Is there a disappointed or sad "boo" equivalent in Japanese?
Kind of like:
Person 1: "Hey man, can I borrow 10 dollars?" Person 2: "Nah I don't have any change." Person1: "Boo, now I don't have enough to go to the concert."
I think you could say 「トホホ」. It's a lot like "boo hoo". I've heard that it's used more in fiction than in real life, but I think that's true of "boo hoo", too.
Chocolate pointed out on chat that you could also say 「あーあ」. Although I don't see entries for this interjection in any Japanese dictionaries, you can find it in Japanese-English dictionaries such as edict. Edict describes it this way:
Another related word is 「がっかり」. Again, I'll quote edict:
Edict lists more parts of speech for this word than Japanese dictionaries, so I decided to link to it here. You can say, for example,