it's probably been asked before, but can i pay the cost for playing the Prison's effect by stunning an already stunned Arkham character? my assumption would be yes since you're not restricted to targetting a non-stunned character. thanks! :)

You may not pay the cost by stunning an already stunned character. The difference between stunning as part of the cost and stunning as part of the effect is that the cost MUST be paid, which means that a character must go from unstunned to stunned.
If an effect, on the other hand, directs you to stun a character, you may choose a stunned character as a target. Blackgate Prison does not fit this as the stunning is a cost, not an effect.

ok, wrong assumption! thanks again for the clarification Dlanaan.