I think you should just be truthful. Only your parents can really tell you the answer to questions 1 and 3. I think for Q3, you should answer with whatever number you are SURE they will provide, even if you get funding from the University. For question 2, it might be hard to quantify but you can ballpark it at about total cost of tuition + rent + food for the last year, which I find to dominate all my expenses.
I am not applying to Columbia, but other schools that ask these questions put them in a section that is labelled as "optional" or "will not affect your admission decision". If it's the former case, you could also not answer them. From an international student perspective, a lot of the forms say I can answer these questions after being accepted. I think they need the information to process visas etc. For domestic students, maybe your parents' assets determine how much tuition you pay?
Some other applications asked me these questions because they want to know if I would require funding from the University or if I would be able to fund my education myself. Be honest -- if you are not going to accept an offer without funding, then you might as well make it clear now. In my opinion, because I am not able to / do not want to fund my own graduate education, an acceptance without funding is the same as a rejection to me, and my answers to their financial questions reflect that.