Coolant out of my exhaust manifold

I have a 1998 GMC K2500 and I was working to replace my exhaust manifold with headers and pipe in a new exhaust.. I removed my exhaust piping, and was about to start removing the exhaust manifold when I git a call with my fire department. As the truck was my only vehicle at the time I drove it the 10 miles to my station and then home after the call. On the way back I noticed a white smoke and it started overheating. I stopped and looked under the truck and I had coolant coming out of my drivers side exhaust manifold. I checked the oil and there was no mixing of coolant and oil. I had a friend tow me home cause I was worried to cause further damage. Whats the most likely cause of my problem and what is the ballpark price to fix it if I do the work myself? Thanks, ~Matt~