So you got a quote from someone who was using your site to learn new skills with 2 years ago and now feel it should be applicable to an experienced ASP.Net developer?
The likelihood is he underquoted the job because he knew he couldn’t charge you the full going rate based on his experience level. So for him he would still get paid quite well, and also have a chance of learning a new framework/language at the same time.
I would suggest approaching a reputable software house and ask them to provide you with a functional specification (it’ll cost you to have this produced). This may mean several meetings to flesh out what you have at the moment, and what you want your bespoke software to actually do. But you’ll end up with a document you can then get sensible, more accurate quotes on.
There’s nothing worse that trying to quote on a job that’s all verbal/email as normally things get forgotten or overlooked. Feature creep then sets in and the job will end up costing more than the original quote.