Valve Seals?

I have been told on this forum that my valve seals are bad in my GST, wich causes it to smoke after i let it ide for 10-15 min, how serious is this problem.

It will burn some oil, lower your effective octane a bit (knock), and eventually coat the tops of your pistons in a thick layer of shit that will raise compression and cause knock from the hotspots on the pointy parts.

The question isnt how serious it is, rather at what point is it worth it to you to spend the cash to get it fixed. That I can't answer. If this car is just drive around, it may not be worth fixing until you have other work to do at the same time, timing belt for example. If you are trying to make any amount of power with this motor, its going to hurt you and you might as well fix it. YMMV/IMO/IME/etc :)

How much would it cost me to have the valve seals replaced?

Impossible to say since I do all my own work. It would cost me 20 bucks for the seals, and another 20 perhaps at my favorite machine shop. The rest of the labor would be free. Its basically a head gasket change/tbelt job. I'd say 500-1000 dollars, but hopefully someone has had this done. OR you could just shop around for the best estimate.

I asked mitsu dealer/shop and they want about 1500 for valve job..... I am in the same boat.
I am having jake/eclipseRST rebuild a 6 bolt and install it.
I will be dropping it off on feb 26th and can barely wait. It will cost about $2500 for everything from him....

I did ARP head studs, and bought a full engine gasket kit. I only replaced the head gasket and valve stem seals, and of course miscellaneous gaskets like the valve cover gasket, etc. If you can get a friend to help you out, you can do the same thing I did for $200 or so, and have a stronger HG afterwards.