Let's use 50 cents as the price of the item Two for the price of one = $.50 for two (or $.25 each) Three for the price of two = $1.00 for 3 or approximately $.33 each Buy one get 25% off the 2nd means $.50 + (.125 which is 25% of $.50) for a total of $.625 or about $.63 ($.315 or about $.32 each) Buy Two get 50% off the 2nd means $.50 for the first and $.25 for the 2nd for a total of $.75 or $.375 - about $.38 each
We can see that two for the price of one is the best value since you save $.25 off of $.50 (that's half) and that buy two get 50% off the 2nd is the smallest price reduction since the $.50 item only goes down to $.38 or twelve cents off. This would work no matter what the cost of the item. The savings would still be in the same proportions.
