When one can save a few pennies or even twenty five percent on the cost of a gallon of fuel, that sounds great...until one burns half of the tankered fuel just to move it from A to B. In that case, the effective cost of the fuel you have remaining...that is, the fuel that wasn't wasted getting it from A to B...has just doubled. Save a dollar or a euro or a pound on a gallon of fuel...but have half of the tankered fuel remaining...and you haven't saved that dollar...you've doubled the cost on the fuel instead. Why? You just paid a little less, but got half as much. That's not economy.
Further, the airplane climbs slower, uses more runway, lands heavier, has a lower climb gradient, lands faster, may require higher power settings and thus experiences greater engine wear, yada, yada, yada.
You tanker 10,000 lbs, a purchase of 1,500 gallons. You do this because the destination has fuel at 4.00 a gallon, whereas your departure has it for 3.00 a gallon. This sounds good; you'll save 1,500 dollars on the cost of the fuel. You make a five hour flight, consuming five percent of the surplus tankered fuel, per hour. The surplus tankered fuel is 10,000 lbs, meaning you consume 25% of the tankered fuel during that five hour flight (5% per hour for five hours).
Fuel at departure would be 4,500 dollars, whereas one would have to pay 6,500 dollars at the destination for the same amount of tankered fuel. Unfortunately, because 25% of that fuel is burned on the way to the destination (fuel burn has gone up due to carrying the extra load), you have only 75% of that fuel remaining...1,125 gallons of tankered fuel. Dividing the total cost of the tankered fuel you bought, 4,500 dollars, by the amount of fuel you have remaining (1,125 gallons), you find that for the fuel you've got, you really ended up paying four dollars after all. You've saved nothing, but instead dragged ten thousand pounds of fuel across the country or internationally, for nothing. That's ten thousand pounds of revenue paying payload that one could have carried instead...or at least 75% of those ten thousand pounds...7,500 lbs of payload that would have paid and been profit.
Break even point? It's not nearly so simple as looking only at the cost of fuel at departure and destination. One needs to consider numerous other factors, including the purpose of the flight, the payload available, the conditions of flight, etc, when making such a decision. Very often tankering provides no additional benifit economically.
We do plan fuel stops where the difference in cost justifies the stop, but in that case we're tankering nothing; we carry the proper amount to reach the tech stop, then take on the proper amount to reach the destination. Even then, merely making the stop for the sake of fuel cost is more complicated than the raw cost of the fuel. Time lost when the aircraft could be on another trip making money, additional trip time and time on airframe and engines, landing and engine thermal cycles, etc, all must be factored into the justification in making the stop. Most of the time, a tech stop just for the cost of fuel isn't justifiable.
Where a tech stop must be made due to the length of the trip (which for us is sometimes the case), we'll break up two six or seven hour legs with a fuel stop selected for it's convenience to the route, it's minimal turn around times, and yes, the cost of the fuel.
You see, in the real world, it's not quite as simple as your world.