I just finished a DIY concrete slab today which saved me about $2,000 compared to hiring it out. It took me way more than 20 hours to prep and pour so that wasn't $100/Hr but I did learn a lot.
I started the project by renting and mini skidsteer (an amazing tool) and removing the top soil and trying to get the slope right for the water to run away from the house. Once I had the form boards done it was clear I'd removed more gravel than I needed to but I'd already returned the skidsteer. I thought about just ordering a couple extra yards of mud and having a thicker slab... Fortunately badassity overcame laziness and I decided I shouldn't spend $150 a yard on concrete when the practically unlimited supply of gravel in the yard would be just as effective. It took me about 4 hours to dig up, haul, and spread about 3 yards of gravel. Excellent exercise too!