building platforms in “loft” space

Endless discussions on gut reno costs, I’ve seen.
So what about the costs of building modern mezzanine inside raw space?
My space has 13 ft ceiling heights, so what kind of budget to build out an upper level of, say, a spacious home office type room and perhaps catwalk alongside new built-ins – one with proper safe access (not ruthless metal ladders), modern materials, architect designed – something that is good enough to be add value come resale, not be ripped out?
If anyone has done something like this, what was reasonable as a budget, 50k? 100k? 200k? was it harder to get the right people than you expected?
thanks

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