I think for MLB it was roughly 90mph pitch connected well with 90mph swing, exit speed of 105 or 110 or something like that and ball hit at good angle travels 400'.
The drills are pretty simple. 40 dry swings then 60 swings to a ball off a T twice a week with a bat -7 to -8 oz lighter then her primary game bat. The trick is finding a bat relatively close in length but still weighs less. There are some specialty products for this that run about $40, but we were able to find something at play it again sports that works. The important thing here is that the kids actually do their homework, it isn't enough to just to do it at hitting lessons.
Here's a link to a Candrea skill list for under loading if you want to mix it up a bit. My DD's hitting coach experiemented with a lot of this and found that just doing the swings above several times a week achieved the same speed increases as these drills, but some people like to be complex for the sake of complexity, so here's the link: Developing Bat Speed by Underloading By Mike Candrea and Jerry Stitt
-W
Agree with Tim. Plus a good hitting coach should use one. It will tell you if what you are teaching is working. It was used to prove the grip change of a device like the bat jack works. Otherwise it is just someone's opinion.