many times that's the largest amount of........
"Many times," Shroom? Why are you pulling your punches? Or maybe there is another service that is more labor-intensive or requires more in serving time and presentation?
Many people, even some pros, don't realize what it takes to serve an appetizers-only event. They are fussy, take a great diversity of ingredients, require a great amount of attention to detail, etc. etc. All that translates into labor time. Add in the direct costs, and the overheads, and the serving costs, and so on, and an appetizers-only event can often be much more costly than even a sit-down plated dinner.
So, as you imply, the OP's first job is to sit down with the client and determine exactly what is wanted, and why. If they're looking for an inexpensive event, there are probably better ways to go.