You can get a standard rectangular 5 gallon glass aquarium with hood and adjustable 50w heater for about that. The tank will be about $10. Hoods with lights will probably run you about $20-$30 but you can get a glass top for $12 and if you're not keeping live plants the light is completely unnecessary. A good adjustable 50w heater will run you between $20 and $30. Good ones are Aqueon Pro, Jager and Marineland visitherm.
You might be able to find good kids at your local petsmart or walmart. If you use one of those filters though please add a prefilter to your intake tube as bettas can get sucked into them.
You want wider as opposed to taller. The hexagon would not be the best choice. I like this one myself: http://www.petco.com/product/14978/P...sultRedirect=1
prefilter it or replace the filter.