Because the nickel will chip and flake off if you attempt to engrave the gun, the engraver will usually send the gun out to have the nickel stripped off.
After engraving, he'll send it back out to have a new nickel finish plated on.
Some engravers may have the equipment to do the stripping in their shop, but due to the problems with getting a good plating, most will send the gun to a professional plater to have it re-applied.
After engraving, most will offer on request "inking" of the engraving.
In this process an ink type material will be put in the engraving to highlight it and the parts will be baked to "cure" the ink.
This makes the engraving "pop out" better and adds to the appearance.
Cost of the engraving depends entirely on what you want and who you pick to do it.
The more coverage the higher the cost. As example Colt offer engraving in grades of A, B, C, and D, with each level covering more of a percentage of the gun. "D" is full coverage and costs the most.
Above "D" from Colt is signed engraving, done by one of Colt's top people who sign the work. Other than including gold or other precisious metal inlays, this is the top of the line.
Cost also depends on who does the work. An engraver with little to no reputation will be cheaper than a nationally known Master.
Last, the quality will count toward cost. A poor engraver will usually cost less than a Master, and will usually look bad to anyone who knows engraving.
One option to boost the value would be to have the Colt factory do the work, since factory work will be worth more than >comparable< quality work done a non-factory engraver.
Last, you have to decide what style of engraving you want. Typical choices are American Scroll and the styles done by Colt's famous German engravers of the 1800's.
The only limits on engraving are how much you can spend.
Colt's web site has full details on their engraving services:
Custom Firearms