In a perfect market, even Yale should cost no more than $100k (EV of a T14 law degree in the first year, or whatever it would be for each school)-$45k (what an average libarts major would get), with the former number upped by 5% and the latter number by 3% over 40 years, at present value at a 4% discount rate. Whatever that number is.
I am not sure why you are only considering the EV(of the first year) and likewise only subtracting the expected salary of the liberal arts in the first year. You really should be taking the Present Value of the stream of your possible outcomes over the length of your law career accounting for the risk of those outcomes(say just for argument sake 50% chance of biglaw + 40% chance of small firm + 10% chance of unemployed). It's not like the value of your Yale Law degree, or any law degree disappears after the first year. You also should be taking into account not the average value of of a liberal arts degree, you should be comparing it to options you personally could have had instead of coming to law school. This calculation will necessarily have to be personal.
I agree that this method requires many assumptions, and lots of data that you may or may not have (like for example the assessment of the risk)... but in theory that's closer to what the tuition "should be" in a perfect market. Also I guess this way of calculating assumes no benefits outside of salary (like the value you ascribe to being a lawyer, whatever that's worth), and no costs outside of the "opportunity cost" of foregone salary of your other job.
Then again, this may very well be not be a perfect market exactly because of this lack of information out there. I personally think that because of the lack of information, and the amount of negative information, you should use low-end estimates of placement to account for this uncertainty, if you are doing such a calculation.
Anyways for the sake of this thread, I will likely have close to 170k in total debt (both UG and law school) when I leave. It's a scary thought. But I do have the benefit of a good LRAP program to help mitigate these fears to some extent.