Iphone 4S: Lock button doesnt work and warrenty is over

My lock button suddenly just stopped working, its been like this for a couple of months. i know somewhere in settings where you have that bubble and you can lock it from there but i already have that. everything is updated and installed- i rebooted it and restored it and still no results. i just want to find out how much would it cost to repair/replace it? when the phone shop my warranty was over, they said i could take it to apple and they can replace it.. can i still do that? how much would it cost.. about 250$? or..??

(btw im in sydney australia)