Is free cheap enough? I use an audio editing program that has a time line that measures to .001 second. It is called Audicity and is freeware. If you put it on a laptop and record your shooting session, then you will be able to find the exact length of time it takes to get a shot off. That assumes that you use a sound as a start signal.
Just my 2 cents, which is 2 cents more than the program will cost you.