Check out some of these diffusers-they seem to be fairly inexpensive and most likely in your price range -
http://www.google.com/products?client=safari&rls=en-us&q=CO2+diffusers&ie=UTF-8&oe=UTF-8&um=1&sa=N&tab=wf&oi=property_suggestions&resnum= 0&ct=property-revision&cd=1; There are other options, but I'm trying to keep it simple. I think you would be happy with a glass diffuser. Sorry I didn't respond earlier!!