I had one done at the local gun plumber back in '09.
I sourced a 6" King Cobra barrel from here on CGN. $175 iirc.
Had the current owner drop off the pistol to the plumber.
I dropped off the barrel I was able to source.
$120 and 3 weeks later I picked up my re-registered pistol.
The $120 covered the cost of the barrel swap, verification and transfer.
This was before Colts started to demand a premium, so I have a very nice stainless King Cobra for a grand total of about $750.
It would be considerably more if a barrel blank was turned and installed!
Cheers