You'll have to send it to either Colt, or a Colt-qualified repair service like Pittsburgh Handgun Headquarters or Cylinder & Slide.
No one can give you an even ball-park idea of cost until an expert can examine the gun and determine what's actually wrong, and what it needs to correct it.
Colt and the other two will send you an estimate before doing any work.
FAIR WARNING: These old style Colt actions really are "different".
There are VERY few people around who still understand them and really KNOW how to repair them.
Take it to most any local gunsmith, and you can kiss it goodbye.
They may TELL you they know all about Colt's, but very few do, and in trying to repair the gun, they usually wind up causing more trouble by altering parts that aren't the problem.
The old style Colt action is a very complicated design that has a reputation for being a "watchmakers gun".
It is NOTHING like other brands of guns, and just because a gunsmith is expert on S&W or Ruger revolvers, means nothing when working on the Colt's.
Bottom line, either let the real Colt pros look at it, or hang it on the wall.
