I'm asking this because i have a gym prerelease seadra i got it from a
guy at a card store and i want to know if it's going to come out and how much
is a prerelease gym leader sedra costs.

The only prerelease card that is worth any real money is the Prerelease Clefable.  All the others were printed in such high quantities and so easy to obtain they are actually worth less than their non-Prerelease counterpart.  I would say roughly $6 - $8 on the Seadra if you were to buy it from a store.

Scott