Either you are being bluffed or you are competing against a hack.
Can you get the specs on the "custom" cabs? How thick are the face frames, what types of doors and drawers, good hinges and slides, all wood or not, hardware included, rollouts included, warranty, stain & topcoat vs. paint, etc.
In cabinetry, I have found it is almost impossible to go apples to apples. Everyone is a little different.
The custom shops are usually higher, but worth it if you are offering installation too. (no filler strips etc. you know the drill)