Hard Goods make lovely concrete and steel chairs. They don't reveal the concrete mix that enables them to be so thin, but Design Milk mentions it is an "engineered concrete". This is usually code for adding pozzolans and fibres and balancing everything perfectly.
The owner of Hard Goods, Brandon Gore, has a great statement about cost and quality on his own website
http://hard-goods.com.
“As with all things in life, you get what you pay for. My price is a direct reflection of the quality of work I produce. If budget is your deciding factor, then I am absolutely not the right person for you. I am not the WalMart of craftsmanship. However, if you value expertise, distinction, and knowing that you bought the best, then I’m the craftsman you’ve been looking for.” I couldn't agree more.
Thanks to designmilk.com for their article here.