Greetings,
This is determinated using the K factor or transport factor, equal to delta takeoff/delta landing in terms of weight, having said that the weight IS A determining factor because we order fuel in weight but it is supplied in volumes and the relation is the SG, now lets say that you are going from a cold country (SG around .80) to a warm country (SG around .78) so for each liter of fuel loaded you would loose .02 of weight and if your total Fuel is 40T.....
think about it.
If the SG are equal then the price difference IS the determining factor
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To recap in all case PRICE DIFFERENCE has to be CORRECTED by SG Difference and turn into a percentage, that percentage has to be less than the extra fuel burned usinf the K factor