Erik, in theory, your argument makes perfect sense. The problem lies with the individual customer, and his or her aquaintances. The job will almost assuredly be attributed to the OP, regardless of the fact they didn't supply the finishing. If the finishing is poor, that reflects terribly on the builders reputation!
I have zero issues telling someone that they should be looking elsewhere if they need a "deal"
I can't continue in business, and provide for my family by giving anyone a "deal" .
I quote the whole job at as fair and accurate a price as I can.
Every bit of my work at this point in my career comes from my reputation - good or bad!
I prefer the jobs that come from having a good reputation. I've taken a lot of my time and money and invested it in my business, and learned what I need to know, sometimes by losing my shirt, and pants in some cases, by trying to do the job properly and professionally. I've built my following of customers one dollar, and one person at a time.
I'm not about to let an inexperienced non professional- doctor, lawyer , or otherwise tarnish or ruin what I have invested my life so far in building up. Nobody who takes their career or practice seriously would either.
It may not be much of a business, but for good or for bad, it is my business.