It didn't cost a whole lot. Here's what I know:
Fan: 120mm ball bearing PC fan. I can get the model number if anyone is interested, but I have to take the stir plate apart to do it. No cost- had it.
Case: Radio Shack model # 270-1809. $7.39
Power supply: 10V, .740 amps. No cost- had it.
Potentiometer: Radio Shack model # 271-0265. $4.29
Switch: Radio Shack model # 275-635. $3.69
Magnets: Ace Hardware model # 07046. $8.95
Washer: No cost- had it.
25ohm resistor. No cost- had it. I have this hooked up the the toggle switch so I can either use it or not. The pot gets warm without it when the pot is on low.
This is the washer that I glued to the fan to hold the magnets. I put it in my lathe to true the OD and ID. I then put it in my milling machine to cut the pockets for the magnets to ensure that it would be well balanced and the magnets would be the correct distance apart for the stir bar that I was using. To figure out the distance, I stuck the magnets to the stir bar and measured how far apart they were.
I did notice how shi*ty stir bars are during this project. Either the magnet's N and S poles are not perfectly centered in the bar or the ring in the center of the bar is not perfectly centered between the N and S poles of the magnet. Good thing the pockets in the washer ended up a little big. Because of that, I'm able to tweak the magnet placement so the bar spins on it's ring. It hurts balance, but at the speeds the stir plate will normally function it's not a big deal. Here's the washer: