By far the biggest question people who are new to the diy cnc world have is about the cost of building a cnc machine yourself. Unfortunately there is no straight answer since almost every person who ever thought of building a cnc machine themselves did so for a different reason, with a different budget, to fill a different need.
We simply define diy cnc projects into three different sizes, small machines range in size up to 24" medium machines typically range in size up to 48", and large cnc machines are anything above 48". The biggest factors for pricing your diy cnc machine (other than time and labor) are machine size, build material, and finally the electronics kit that you will need. What you build your cnc machine out of ultimately depends on its intended use. A machine that is going to be used to cut 1/8" plastic could be made out of wood or even plastic. While a machine that will be used to cut metal will need to be made on a heavy metal frame.
So here it is the answer you have been waiting for as a general rule with plenty of room for argument:
Small machines: $400-$1000
Medium sized Machines $800-$5000
Large Machines $2000+