I believe it is 25% of production cost. So a 10 shield warrior yields 2 shields when disbanding, a 90 shield infantry yields 22 shields when disbanding. The experience and health level of the unit has do not matter. An injured conscript yields as many shields as a full strength elite.
Disbanding is immune from corruption. Some players build bombers (100 shields to produce, 25 to disband), relocate them, then disband them to get production in distant cities to produce buildings. This is a costly, but effective method.
Be aware that disbanding does not help in construction of wonders. Firaxis really made wonder construction a tough nut in Civ III. The only exploit is to start construction on a Palace or another wonder then switch to the one you really want when the tech comes.