Like in the modified car world, it should be cheaper usually (or should be cheaper usually) to pay less for a modified item, than to buy stock and modify it yourself. Not that it should be cheaper than stock, but you typically never get back what you put in.
Then the question comes down to who did the work and I was it done well. Some things, like a simple drop in modified trigger I think would add maybe 80% of the cost of the trigger, to the value of the gun (depending upon how long ago the mod was done). Extensive work still has depreciation. The amount of which depends upon the opinion of the buyer and quality of the workmanship.
I enjoy modifying my own guns in the same way I enjoy modifying cars and trucks. Some folks want the mods but don’t enjoy doing it themselves so they either pay to have it done or buy modified.
I think the bottom line is unless you are in the business of selling new modified weapons, you will almost always loose a little money selling a modified firearm of what you have in it.