The cheapest fruit fasting is NOW! The grapes are ready and there is no better/easier fast that to fast on grapes and or grape juice. One can eat as many fresh grapes as they desire or drink the juice. I would pick my own or from a local market ONLY and do not fast on imported grapes that have been heavily sprayed.
Next, coming up are the apples, soon in October the finest Apple Juices will be made at the local apple orchands. Those that can get the juice raw have one of the finest fasting juices available and a super cheap price. Both apple and grape juice fasting has been written about by authors of the past; they don't seem to have medical history like oranges did, but still just as good.
Local juices all work well, peach/pear/etc. would all make great fasting fruits. Even watermellon is excellent and what can be cheaper!