Estimating repair costs on fixer-uppers

So we are just getting started, and our budget will only allow us to buy fixer-uppers.  Prior to making an offer, and getting (and paying for) a formal inspection, how does one get an idea of what the repair costs will be?  More accurately, we don't just need to know how much it costs to do X, Y, and Z.  We need to know which of X, Y, and Z is needed, because if it is major, it may be unmanageable and not worth it for us.  What do you guys do?  Thanks!