My beloved 84 Trek 850 MTB (lugged, 531) was in a collision; causing a buckling on the down tube right below the joint at where it intersects the head stem. It also caused a slight tweak on the top tube and pushed (but did not bend) the front fork back about an inch. Though the bike still rides fine, my bike guy said it's toast and would cost more than the bike was worth (it WAS worth about $400) to try and repair it and even then, a re-weld would not be as good as new. Sound about right?
I agree that having the tubes replaced will cost more than the frame is worth. It is sometimes possible to pull things back out though. It depends upon how bad the damage is. It will never look good again, and remaining strength/safety will have to be determined by you. I once had an old road frame that had been subjected to this treatment. I had cantilever bosses brazed onto it and use it as a cyclocross frame for several years. I eventually gave it away; as far as I know it is still going.