Interesting work from Raj Chetty, John Friedman, and Jonah Rockoff
The earnings gains from replacing a low value-added (bottom 5%) teacher with one of average quality grow as more data are used to estimate value-added. Discounting future earnings gains to present value, the gains are $190,000 with 3 years of data and eventually surpass $250,000 per class. If future earnings are not discounted, cumulative earnings gains surpass $1.4 million per class.
You should see the three graphs they show on their summary page.