COL is usually overestimated, but that's because of how I look at it. When I think of COL (and apparently I am in the minority), I generally think of the cost to get by, not to live well or in a decent neighborhood, etc. If you're going to school and taking on loans, I'd recommend eating Ramen at least once a day and making your own meals. If you skimp and do what you can to save money, you can get by living on your own under 15k/yr in most areas, even cities. If you're going to law school in a small town or rural area, you can probably do it for 10-12k, but that's if you're really living on the cheap, and not including what should be one time purchases, such as a suit.
As jeremydc said, 20k/yr is reasonable for a "semi-comfortable" lifestyle in most big cities, 15k for small cities. That seems accurate to me. Ultimately, its up to you. If you're okay coming out of law school with an additional 10-30k in debt to live more comfortably than is absolutely necessary, then that's your choice to make. But debt just sucks - I'd recommend avoiding it if at all possible, especially unsubsidized. In all likelihood, your expenses will come close to what you'll find listed. I'd check lawschooltransparency which, for example, says the COL at BU is 18k, NYU 27k. My only point here is that you can do it for significantly less - CAN. Eat Ramen at least once a day, take good care of your clothes (I've seen people dump money because they ruin nice clothes), make most of your own food, and live in a cheap apartment. But that's a value judgement - some people would rather take on an extra 30k in debt during law school to improve their quality of living, some would rather minimize debt at all costs. For the most part, I think, it depends on what you want to do out of law school. If you've got biglaw lined up, living it up a little bit isn't so bad, but if you want to work in public interest, you better be eating Ramen twice a day in law school.