I guess it is how well you survey prior to the work.
I had two examples.
JPC surveyed our boat and quoted for fitting a three outlet 3.5kv Webo. We agreed the price but on fitting they found the stringer they had planned on going through was just too thick so they came up with another solution. This solution took more time but they did not quibble on the original price. Their view was they should have seen this. The upshot is I would recommend JPC to anyone that asked as the workmanship and attitude was first class.
Second example Goodchild quoted me for fitting a Raymarine Autohelm, quote accepted and actioned. Top class installation and when the bill arrived it was under the quite as the job had not taken as long as anticipated.
I guess with both the above I had full confidence in the companies that were carrying out this work, I would certainly not trust our ship to anyone. The old adage relating to if it looks too cheap to be true it probably is I would suggest is what to keep in the forefront of you mind. I am never looking for a 'cheap job' I am looking for value which to me means competitively priced; top quality work, right first time.
Check how qualified the person you are thinking of engaging to carry out the work is.