There's a lot of factors that go into the cost of a costume besides the fabric.
If you don't have a large stash of notions and tools, every single little thing is going to cost money - buttons and other fasteners, thread, needles, a new scissors maybe, tape measures, and so on. Then there's the decision factor - how you decide to make each part. For example, are you thinking about making the jacket as a corset? Do you want to make book covers or completely custom footwear? How things are made affects the costs of materials that go into them. Also factor in underpinnings - you may need a real corset underneath the coat, which is going to take more fabric but also boning, eyelets, probably an eyelet-setting tool, etc.
Start by figuring out what patterns you would use for each part of the costume that you need to make, and then use those to estimate how much fabric you need. Don't forget linings and contrasts and other things. You can easily research different types of fabric and how much X number of yards of different kinds will cost you, with and without coupons or sales. Then factor in whether you need things like piping or trim, eyelets, buttons, etc. If it's already starting to be a much larger number than you wanted, even without considering any new tools you may need to buy, base shoes, a wig, etc, then you have your answer.
Founder and Vice-President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com