If you pick a pattern with a CNC or laser engraver, you can usually get them done incredibly cheap. Custom set ups often cost more - not surprising, they take time.
Get custom hand engraving done, and you start talking about EXTREME costs for very little work done. $100 or so to do the backstrap of a revolver, maybe $800-1500 to get full coverage on a 1911 done.
Just need to figure out what you want done, engraving, CNC engraving, or Laser etching, then call up the respective folks and get a quote.