As a welder people ask me all the time to quote em on a project from a picture found online..... My answer is always: If you want to see if I will quote for less than what you found online don't waste your time ( And mine..)
Custom work is really hard to quote ! All you can do is to compare with an other project you've done before. Other thing to take in consideration, sometime you quote a little low because you feel it will be a returning consumer. If you just starting you can always tell yourself that you are learning.
Quoting a job is like telling a girl "I love you" you never know what's gonna happen next lol !