A deer antler is harder than a raw bone, softer than a cooked bone. It also depends on the antler. Some are more dense, and some are more "spongy." I go for the dense ones because otherwise Gavroche kills them in no time.
I get mine from my trainer for $5 for a good sized piece or $10 for a quite large piece. I generally use the $5 pieces. Check if there are any independent distributors, hunters, etc because everyone else marks them up like mad. The pet store here sells them for around $5+ an ounce.