Finzolas,
I am slightly surprised that you think this is difficult. For a pilot down trip it is true that the various rules of thumb and back of the envelope calculations make it an inexact science. But since you say that you are involved in the back-office planning for a 737 operator, you presumably have access to all the data you could possibly need to make this a highly exact science.
Assuming you have access to some proper performance based planning tools, you just run the trip plan for every trip based on (1) the intended payload and (2) a payload 1 ton higher. The second trip plan will take a little bit longer and wil cost a little bit more, and the difference is what people are calling the "cost" of tankering a ton of fuel. You can then compare this to the cost of a ton of fuel at the next stop, and immediately see whether it is of benefit to tanker the fuel.
Like all these things it is dependent on having good initial data to get good answers. If you flight planning program incorrectly calculates the true cost of a flight, then you will not get the right answer - but you should probably be fixing that anyway for the sake of the overall operation. And, speaking from personal experience, if you make a mistake on the actual price of fuel at the two locations, you can wipe out the whole benefit in one horrible mistake.
But on the assumption that you are starting with good data, then it can be very satisfying to scrape the odd 50 euros here and there on every sector, and for a busy operator that soon justifies the original investment in the flight planning software, the fuel price database, and the Excel spreadsheet to tell you the answer.