He's getting downvoted, but he's not wrong - however, finding someone who specializes in doing this would be hard, and knowing how to word it so you can use "standard crews" would itself be $2-3k for a GC.

This is the danger of "cheap projects" - they can grow legs before you know it, and you end up paying half the cost of increasing the square footage of the house (or the garage).

Yeah of course you could DIY this or find your own subs much cheaper, but in the grand scheme of things, it's not that bad.

You get a finished and conditioned room, and it most likely added value to his house.

Unless he’s a commercial builder, I’d suggest he is wildly wrong about what is involved in properly pouring, leveling and making sure concrete has properly set in various weather conditions. And if you take a look at his post …

Some things are deceptively complex (foundations and roof trusses come to mind) and the cost if you get it wrong can easily double the total cost of your build 6 months after you think you are done.

Walls, floors, ceilings and everything in between them are both less complex and cheaper to fix than foundations or trusses.

It's a shed, you can just compact gravel and put it on pavers