In that case - first see if the hub is disc ready or the rim brake version, some people chose to have that hub even with rim brakes to future proof it. If not it needs the shell cap replacing which is a workshop job. Then if it's an internal shift mech it needs converting to external (Can't fit a disc with an internal), you can do that yourself with the parts, but if you're sending it off for the cap you may as well have that done in the workshop. Cost of those two items was around £350 seven years ago, don't know what the current price would be. Then of course you have the cost of the rotor and brake, plus the frame may need facing. Then it's just the wheelbuild.
I went through it when I put the hub that originally came on my Raven, into a Mercury frame, SJS did the work. If I had my time again I think I'd have sold the hub and bought a disc version new, there wouldn't have been much difference to the cost.
