are you sure you're wanting engraving ? most such work is usually cast bronze, brass, even aluminum. all with a nice patination to complement and protect the metal. i have used "mathews bronze" co here in the states for such work. easy for me, as a facility is within driving distance from my home. most monument companies can steer you to a source for this work.
nothing against engraving, but the casting process makes for a far better end result. the depth makes such work really stand out !
I once saw a plaque on a monument outside of a high school in Bisco, NC (I think) that Jessy Hauser engraved with a air hammer ... it was huge. They cast the plaque out of some sort of bearing material (Babbitt maybe) and he made an engraving tool that fit into an air hammer and then proceeded to engrave the plaque.
You can't cast something without having a pattern that someone has to carve first. Enough information hasn't been provided. Perhaps he only wants lettering engraved on the plaque. If so, there are other alternatives.
Also, no one can give a quote without knowing complete details of the scope of the project
I was not suggesting that this was the answer to anyones request for a quote ... only stating what was once described by artist and what I saw ... FYI the plate was first cast (think flat sheet) and THEN was engraved.