This is a list of the materials I used. I'm not selling these things, I'm just giving a little helpful advice.
1 Computer fan of the 12 VDC variety 120 mm about $7.00
3 Rare earth magnets 35 lbs per pull, 1 for the keeper and 2 for the fan. Cost $3.35 each
1 Stir bar Cost $5.00
1 Potentiometer, 2,000 ohms Cost $5.00
1 LM317 voltage regulator Cost $3.50
1 resistor, 330 ohms 4 pack Cost $3.00
1 capacitor, 0.1 mf 2 pack Cost $$3.50
1 Project box. Cost $7.00
1 12 volt transformer Cost $3.50
1 plastic dial for the pot $3.50
That's a total of over $50.00 You can get one made and sent to you for that price.
Without the right resistor, capacitor, voltage regulator, and potentiometer you won't be able to control the RPMs and you'll throw the stir bar to the side of your flask.
Now I know that some people have some of this stuff laying around so you might as well put it to good use. Also, these are Radio Shack prices so if you hunt you can probably find it cheaper.