BookMooch: To mooch or not to mooch?

I've been thinking about joining BookMooch for a while now; I actually do have an account, but with nothing on it. My main concern is shipping costs--hence, I ask the almighty flist! If I did decide to participate, I'd only ship within the US; so, considering that I live on the East Coast, what are the approximate/average shipping costs for mass market, trade, and hardcover (respectively)? I'm not sure if cost increases with distance if it's domestic, but you can figure shipping to California for the purposes of this question.