This study says 3 mph:
"According to the study, Kinematic And Kinetic Comparison Of Baseball Pitching From A Mound And Throwing From Flat Ground, G.S. Fleisig, R.F. Escamilla, S.W. Barrentine, N. Zheng, J.R. Andrews, American Sports Medicine Institute, Birmingham, AL 35205, the difference in stress on the arm from throwing off the mound versus flat ground is as follows (first number represents defeat from mound mound/second number represents 60 ft. on flat ground)
Arm cocking phase:
Elbow varus torque 54 ± 7/51 ± 8,
Shoulder internal rotation torque 55 ± 10 /54 ± 10,
Shoulder anterior force 330 ± 40 /340 ± 70
Arm acceleration phase:
Elbow flexion torque 52 ± 7/52 ± 8
Arm deceleration phase:
Elbow compressive force 800 ± 90/780 ± 100,
Shoulder compressive force 910 ± 110/890 ± 110,
Shoulder posterior force 360 ± 200/350 ± 150
Instant of ball release:
Ball speed (mph) 79 ± 4/76 ± 4
From these numbers we see that there's virtually no difference with between flat ground and mound with respect to stress represented by torque and force on the arm. From a percentage standpoint there is a significantly greater percentage increase in velocity throwing off the mound as compared to the present difference of stress on the arm in other words it appears that can throw with greater velocity with less stress off the mound you can on flat ground. Which makes sense, i.e. the additional dropping distance due to the mound versus flat ground allows more energy to be developed from the falling potential to kinetic energy of the body. Bottom-line, flat ground throwing is neither a significant negative or significant positive with respect to its effect on throwing ability. How and when its used as part of a larger training program, training plan."
http://www.webball.com/cms/page1432.cfm