What is the cheapest half decent Geiger Counter?

I am doing a physics project on background and ionizing radiation, and thought it would be good to demonstrate a geiger counter. Can't find any that are cheap-ish so far. I am looking to pay �30 give or take a few. The reason for the cheap budget is due to it only being for a project. I want to know what the cheapest Geiger muller tube or counter is.