I have been making my own mustard for years. If you buy the powder online ( I get mine from
www.atlanticspice.com) the ingredients cost should run around $1 for about 24 oz of prepared Mustard.
I encourage everyone to work with their own unique ingredients to find a balance as to what you are looking for. I made a Bourbon Honey Mustard last. One thing I will add is that after you mix the Mustard it will be the hottest, and it can take a while to mellow. Hold it at room temperature for a few weeks until it mellows to the level of pungency you are after. Then arrest the process in the fridge.