The headgasket problem on the northstars is because of the heads physically lifting up off the block.
Your options are as follows, sell your DeVille off for $1000 - $2000, or you can fix the headgasket yourself with a set of northstar performance studs for about $1000 - $1500 depending on what you want to replace other than the gaskets, or have a dealer do it for $3500 - $5500