Kill House Airsoft talk about the Mancraft PDiK HPA engine as the cheapest in the market... "This is the cheapest and simplest HPA engine on the market it's the Mancraft PDiK engine. just for £260 inside a gearbox for you to drop in your existing Airsoft RIF and turn in to a HPA beast. HPA has its pros, easy to fit and maintain and less to worry about like and AEG and at this price its hard to say anything negative apart from no full auto."