you have to pull the trans, send it out to be undercut, maybe replace 2nd gear if wore real bad. Replace shift fork(s) if neccessary, billet shift shafts arne't a bad idea $60 or so for them.
depending on where you go $300-$400 on trans undercut, $60 in shift shafts if you go that route, $65 per shift fork I think (3 total, maybe you need just 1), $40 for a lower end gasket kit by cometic, plus labor.
let me know if you need any parts i can save you a few bucks. I am going to do this on my wifes bike this winter, my bike did it years ago, tons of them out there with this issue.