This is my experience in making a Batmobile Transformer Costume. Total material cost for this project was a little over $150. It took 2 months of planning and 1 month of construction to finish so make sure you are serious about a costume like this before you start.
The first thing you have to do is decide what you are going to transform into. I chose the Batmobile Tumbler. It is easier to start with the vehicle form first, then figure out where things are going to move and what your robot form will look like.
Fewer moving parts will make things much easier on you. Try to get a general idea of where your head will be, what position you will be in when you are transformed, where pieces might be attached, etc. You can never be too prepared.
Make sketches to help you visualize it. Be very creative but just don't go too overboard with the moving pieces.