I'll try to keep this as simple as possible.
I work for a company that provides services to other companies within a specific industry. I've been working for the same customer for over 10 years and in that time have amassed enormous amounts of priceless experience for that specific customer, along the lines of, I'm one of perhaps 3 that really understands what's going on.
I'm planning on leaving my current employer and gave the customer first chance at extending an offer of employment; they're desperate to retain me and have already offered employment.
I know what my salary expectation floor is, but I've no idea how high the ceiling is. I know some millwright stuff for fixing mechanical issues, but I'm not a millwright. I know some IT stuff for checking the database or auditing software, but I have no computer science degree. They have payscales for all of these jobs, but I have just enough experience in each of them to exactly fit the line of business but can't claim to be a master of any.
Where does one start trying to figure this?