For less than $100, you can probably hit up a scientific surplus store and get a used Corning or some other scientific brand. Speaking from experience, the Cornings are built like tanks. I stir 5 L on all four of mine pretty consistently and they all work well. Also if you have problems with the stirbar kicking, adjust the stirbar size or try putting a thin layer of styrofoam in between the stir plate and your flask.
Also remember that you don't have to crank it up while stirring. A small vortex and enough stirring force to simply keep things in suspension is more than adequate.
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