You can have cheap or effective, not both.
Em, afraid I have to disagree there. Effective is simply a word that mean something that has an effect. Any insulation you use would "have an effect" i.e. be effective. Also, insulation is not on or off. it's an analogue scale. I don't expect 100% heat retention. That would be a bit tricky. So somewhere between what I have now and 100% would be good.
Effective----successful in producing a desired or intended result.
Cheap option will not do that.
He is working on the mistaken understanding that heat rises, so thinks insulating the roof will be an 'effective' way to improve insulation.