The only CONCIEVABLE way bare concrete is practical is if you live in an always-warm wet climate where you can hose the floor out every day. Although it will still be hard on the chickens' feet and I wouldn't do it personally.
Shavings are not nearly as uneconomical as people tend to think -- they expand from those bales they're sold in, so that one $6 (or whatever your local price is) bale will do a 6x8 coop pretty thickly, or a 8x10 one somewhat thinly.
Then, if you use a droppings board under the roost, that will intercept almost half the daily poo output right there, so it never even affects the shavings.
And, if you use one of the many many DIFFERENT varieties of deep litter type management (you have to experiment to see what strategy works best in your particular situation) it is easily possible to use very little shavings on a yearly basis. I would say that I probably use about 1-3 bales of shavings per year per pen (most are 6x10 ish in size) although please note that I have far less-crowded chicken populations than most people, which helps a lot too.
You can also experiment with "found" materials like dried grass, dry leaves, etc... however you have to be able to get them REAL DRY before putting htem into the coop, unless your coop is largely open-sided, and this can be a problem for many people, as can getting them in sufficient quantity. Or, some people use shredded paper (waste from office, etc) although personally I find that to turn into a particularly nasty form of paper-mache, but, some like it.
Good luck, have fun,
Pat
