When I was asked for a quote to redo wiring on another cruiser's boat, my estimate was too high. Answering the question as to why, I said the good heat shrink was expensive, as was tinned cabling and wire, etc. I just wasn't comfortable with the job, at any price I told him so, nicely of course.
If the job makes you uncomfortable, pass on it. Your mind has good instincts.
I like to use the phrase "I respectfully decline" followed by a brief, but possibly vague explanation of why.