1-In your computer flight plan, check the extra fuel burnt per each extra ton of weight. Most CFPs have that.
2-Then compute the total extra fuel burnt for the maximum tankering you can make taking into account LW and TOW.
3-Then find the factor. For instance, if original trip fuel was 10,000 kg and you need to burn 200 kg to carry the extra fuel, then the factor is 2%.
4-Now you can say that any tankered fuel costs the fuel price plus a 2%. If the fuel at destination is more than a 2% more expensive tankering is advisable.