fist of all, try to find the salaries of assistant professors who were hired in the last few years by the school. Assuming a 5% or 3% annual raise, you should easily figure out the starting salaries of those junior guys. Then, simply add 10% or 15% to that number as your asking price. The final "out- door" price is up to how hot you are on the market, and how much they can afford.
What I heard from a highly respected professor on this topic is, you don't ask for a number specifically. You simply say that you will accept any competitive salary, and if they do care about you, they will make an offer that fits the market.