It depends....on one's definition of "custom".
If a donor factory action is to be considered it must by fully trued including lugs. Not much difference in overall cost than a "custom" action.
I have a 'custom' built on a Rem action and one built on a Bat action. The Bat actioned rig is "more" custom than the one with the Rem.
From owning an attempting to use a lathe, I have learned a bit about how much consistency and a thou is worth. The closer the tolerance the increased skill, patience, feel and discipline are needed. That's where the worth is!
I have "screwed" several rifles together as I consider that I am by no means a "fitter". From the "fit and finish" as is expected and achieved by any smith on this forum I consider my DIY projects "screwed" together.
My DIY rigs will shoot along with my custom rigs for a little while and for the "most part" after tons of tuning, fiddling and shooting.
The one fully, Bat action, custom rig, the process is simply use the load the smith tested it with, cartridge dimensions the smith used (got some loaded rounds with mine
), load sight-in during break-in, develop drop chart and start harvesting stuff.
The seemingly extra bucks actually balance out in your favor over the long haul....
Keep in mind "there ain't no free lunch"