This has been answered before in other posts. However I will answer your question, But in the future try looking around the board a bit more before posting a question. I know I sound like a broken record but many of the most common questions have been answered more than one time.
Now with that out of the way to answer your question. 150-300 US Dollars depending on how much detail and work you want to put into it. Some people on here have spent more than 150-300 but it seems if you break out a calculator and start adding up resin, fiberglass cloth, Bondo, sandpaper, glue, paint and card stock. You will easily make the 150 mark.
And that's not counting the fact that you may kill a printer cartridge and that can add another 20-50 US dollars for a new one depending on model. Also there is the possibility of messing one of your pieces up. So that could add more time and money to the project to get it right.
If these numbers are scaring you the best advice I can give you is this. Do a little at a time. Buy one can of resin and a few packets of fiberglass some bondo and just work on the helmet for instance. If you take you time you can make the armor one piece at a time and not have to spend huge money all at once for supply's. Plus doing a piece at a time gives you the opportunity to wait for local auto parts stores to run sales. That's when I buy my Resin and such when its on sale. Hope this helps you out