Teachers rarely get overtime, but overtime is necessary. The best teachers I know, and I know about 20, work 7-7. They work during the summer to setup curriculum, to tutor summer school students, to select books, to setup their classroom. And they do this all without any extra pay than their salary. They also supplement poor school budgets with their own money to buy supplies the students need to learn.
They are generally pressured to pay into a Teachers union that supports the misfits in their profession, and doesn't generally help them at all. These same awesome teachers are paid solely on the number of years they have been working -- not (at all) by performance, skill, or student/parent reviews. This translates into little reward for awesome teachers over not to awesome 8:15-3pm teachers.
But I digress.
I'd give a multiple of 1.2.
On the other hand, lack of external employment options makes tenure MORE valuable, since it raises the risk premium the teacher would demand to quit the tenured job.
Since my financial math is rusty, I won't try to compute the actuarial values of these revenue streams, however.
To begin with, multiply by (12 months worked by others)/(9 months worked by teachers).
Then add in the value of the state health plan, usually fantastic.
Then add in the real value of a defined benefit pension plan (chances are you only get a defined contribution plan) available in most states.
Then add in the value of tenure, which I'll tentatively estimate as effectively doubling the salary (1).
All that is actually pretty good for a group of people who tend to come from the bottom of the barrel cognitively (http://www.ncsu.edu/chass/philo/GRE%20Scores%20by%20Intended...).
The idea that teachers are underpaid is a myth propagated by teachers unions and their lackeys in the media.
(1) Many tenured professors could easily double their salary by quitting, but choose not to.