If you are in zone 8 with musa basjoo, it should be pretty easy to insulate with cheap materials. You could simply use something such as a combination of bubble wrap covered by shrink wrap which should be enough insulation for your climate. Plastic tarps might work as well.
I'm 2 zones colder, and here's my set up which should work well since I've been overwintering them successfully for the last 2 years.
Steve's garden: Overwintering the bananas and other plants for winter of 2012/2013
If you do anything similar to what I have, I think it would be overkill for your climate, but you can insulate with fall leaves if you are concerned.