I used to shake every hour but the stirplate not only is easier, but makes more yeast. I easily have an extra 1/4 to 1/2 inch on the bottom of the flask after using the stirplate.
About the $10, this is what I did.
Get a 80-120mm computer case fan. Newegg is a decent place to get them on sale. In their email blast they usually put one on sale for 1.99 every week with free shipping.
I just checked and they have a number of them on sale either free after rebate or for 1.99 with free shipping. Even some LED light up ones.
Newegg.com - Computer Hardware, Fans & Heatsinks, Case Fans, Mail-in Rebate
Then you need to pick up a Stainless Steel Washer, 2 inches wide with about 1/2 or smaller hole in the middle. 25 cents.
Pick up some Neodymium 42 magnets at Ace or Hobby Lobby, or buy on Ebay. $5 or less. You can get 35 strength but you may have to do 3 on each side.
Get an old cell-phone charger 9v or 12v $2.99 on Ebay.
I used an old 50 CD-R spindle top to house my computer fan and magnets. That was free. You could pay 5.99 and get a radio shack project box if you wanted it to look nice. Also, others get on off switches and throttle knobs, but when mine busted during installation, I just wired the fan direct to the cell phone charger and have never had an issue.
You're going to epoxy the SS washer to the center of the computer fan motor. After it dries, place 2 neodymium magnets on each side of the center hole. You can epoxy them if you wish, but they should stick in place by themselves. You'll want to space them so the end of the stir bar (1" works good) sits in the middle of each magnet. With that done and spaced, wire up to the charger (yellow line gets nothing), put on the CD spindle cover (you may have to cut out the center a bit if it has an indention) and try to space it so the magnets are as close as possible to the top of the case. They should nearly touch the top of the plastic. That should do it.
Here is mine, it's not pretty, but that could be solved for 5.99. I've done 7 starters on this so far, from 500ml up to a 1.8l last night, no problems.