I guess cheap is relative. 30 sheets at $10 bucks is $300. Make one trip a few miles down the road, pick 30 sheets in new untouched condition, load it up and take it home, done in less than 30 minutes.
Spend months looking on CL, make 10 trips all over the county to buy a few sheets here and there, spend hours and hours collecting sheets, not know what has been done to sheets (no warranty included), might end up smelling like cat pee. How much is your time worth to you?
Some people really like playing the CL game and that is cool, I just don't have time to mess with the waiting and hoping game.
Anyway, I have 3-4 sheets (1/2 and 1/4, in cement and hardi versions) in my garage I would love to get rid of if you want them? I won't even charge you.