The proposal I've received from my current developer is:
They are also charging Accessibility Services: WCAG 2.0 AA
Would love to hear your thoughts.
I do not think anybody can give you an answer on the correctness or validity of that quote without much more research on your current setup and what you will need in the new store. Your developers probably have much more information if they quote $15,000 for Category, product, customer and order data migration.
We just did that for a client yesterday and it took 6 hours but every store is different and agencies charge different per hour rates.
If you want a few quotes to present options to your management, find a couple of developers here on the forum, PM them the details and your contact info (do not post it publicly) and ask for a quote.
Hope this helps.