Looks to be 1% to 3% according to this:
Even though Crappie are HUGE spawners, they are also one of the most delicate....things really have to come together to have a successful spawn...Crappie spawn year after year in the same area/cove/bay/shoreline, if that area changes, it will effect spawns, water temps have to be perfect, water flow or current has to be perfect, low water robs fish of preferred cover, high water release from the lake effects spawns by removing fry in the release, high STABLE water is actually preferred to add nutrients and boost zooplankton for larval Crappie...and then if it is a good Spawn, maybe 1% will survive to legal size (high end is figured to be 3%). Then you also figure that Crappie have one of the highest HOOKING and HANDLING MORTALITY rates if released, and you can understand why they can require management to have a continual Crappie Fishery!
Pulled this from http://www.crappie.com/crappie/archive/index.php/t-299582.html