What your non-electrician "experts" think is not particularly relevant. What matters is what licensed, professional contractors in your area will charge to make the changes. That said, a big difference in the price quoted by different contractors is usually due to one or more of the following:

A poorly defined scope of work.

Differing assumptions about the project scope, quantity, and/or quality of the materials and labor required to complete the scope of work.

Radically different cost structures between the contractors, e.g., a one-man company vs. a large commercial contractor.

Intent to pull permits and have the work inspected or not.

A contractor which doesn't want the job and has put a price tag on it so his/her company is not selected. Alternatively, if they are, they'll do it for the premium price.

Those are some thoughtful rationales. "Radically different cost structures" might explain it. The $15k quote was from a large company who showed up promptly and gave me a quote the same day. The $6k quote, on the other hand, was from a 1-man company who took over a month to finish his quote. The suggestion that these were both too high was from a general contractor who will hopefully be in charge of our addition. He offered to send his prefered electrician our way, so we'll know more soon.

15K isn't even enough for the scope that needs to happen. You need a new service, service drop, panel, and home wiring. Double that 15K and that would be about right. The other guy is high. You are too if you thing that is a 3K job.

While waiting for the "preferred" electrician - google the two who just gave you a quote.