Neither ( quantified financial outcomes or my own technical perception ).
People who think they can build website are often the wrong sort of customer ( or maybe the worst sort of customer ) for web developers and they can generally do without them, there are enough good customers to go around. They ( people of think they can build ) may be successful with their website ( like you perhaps ) and not need help - or they may not be successful and then seek help - but once they seek help and have had experience themselves they are likely select the right sort of web developer for them, and to value the time and expertise of the web developer, and be a better customer.
Your case makes the point, you did it the wrong way round, you engaged a web developer with out knowing how to direct him/her and they didn't ask you the right questions about your strategic objectives so failed to deliver the returns to their potential. So you learned the expensive way. Had you built your site yourself first and learned lessons about visitors and conversion, you could then have better selected a web developer and worked with them to deliver your business objectives, or indeed not needed to use a web developer at all.