You can probably do most of the work yourself and get the slab in for about half of the prices quoted.
At 4" thick the concrete will be ~$1.50/ sq ft. Add in some extra for footers. #4 rebar would be $5-6 per 20' section, 120 pieces would be more than enough. Less than $1000 for drain rock, vapor barrier and sand.
My guess is that it will take you about a week to excavate, put down the drain rock, vapor barrier, sand, and place the rebar.
I would hire a crew for the pour & finishing. For true luxury, hire a helper to set the rebar.