bksptr,
The tranny light is supposed to only light if it detects a difference in gear speeds. The lets the transmission know if there is slippage or the gears are shifting properly. However, that does not always work and many people have had transmission slippage called shift flare, yet the light does not come on....and when it comes on, it often means the transmission has already had a major problem.l..often called the blinking light of death.
If you know the tranny is leaking fluid, don't wait for the light to come on....get it fixed ASAP. As for cost? Just depends on what they think or find wrong?
...Rich