You cannot fly any less expensively than by the PFA route. I got 500+ hours in a little PFA taildragger and had a great time inexpensively accumulating flight experience.
However - (in general) most PFA types are very light aircraft with minimal levels of instrumentation (to keep the weight down) and little systems redundancy. There are good reasons why they are not allowed to fly IFR in the UK. Believe me you really don't want to be accumulating ice at FL120 crossing a front in an aircraft that someone has built in their garden shed and weighs less than a ton. You also probably want to avoid flying in significant turbulence or even pushing crosswind landings because the experience is highly unpleasant as is the bill for repairing the damage of a heavy landing or groundloop. So what I am trying to say is they are loads of fun in the right conditions and will save you a ton of cash in terms of maintenance and fuel costs.
For any serious business flying or IFR activity it really needs to be certified IMHO and preferably de-iced with all the gadgets.
