I mainly use my little Skutt 181 for everything I fire. I know how long it takes to fire for bisque and for glazes. To give myself a good fudge factor I just use the max wattage for the total length of firing, which I believe on that kiln is 4800 watts. And multiply that by what a KWH costs me. You can easily figure the cost of each piece if they are comparable in size.
I use a mixer on my drill. One thing you want to make sure of is if you mix in a plastic bucket that there are no sharp edges on the mixer. I ended up with pieces of plastic bucket in my glaze when I first mixed up the glaze. Good think I sieve it a couple of times after mixing, that's the only way I would have found out it was in there.