Cheapest way is to buy a share in an aircraft. Have a look in the PFA magazine, or on the notice board in your local airfield. A 1/8th or 1/10th share in a small aircraft won't cost you a fortune. The flying, if you pick the right type of aircraft, will be about as cheap as it's possible to get (I used to pay around £30/hour in a 2-seat 120kt turbo-charged aircraft with retractable gear and a variable-pitch prop, for example). And once you've finished your hour-building, you can sell the share to get back most of your initial outlay - unless you get so hooked on your plane you don't want to sell, of course!
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