Uhm, a ton of JetA1 costs, what, $200 - let's also assume that an average EU shorthaul flight is, say, 2 hours, and that the extra burn for tankering fuel is 5% per ton of extra fuel per hour......... so in this instance the cost of carrying an extra ton of fuel is $20.
But heck, let's assume that I've got it way out and double the tankering addition to $40.
Q). Anybody care to tell me what a diversion costs ?
A). It's $0000's !
I.e. There'll be extra landing fees, handling fees, navigation charges, fuel uplift, crew hours, etc.....
E.g. If a diversion had direct costs of, say, $1600, then ( using a tankering cost of $40 ) - you'd have to operate 40 (or probably more) diversion-free sectors ( using no more than PLOG fuel on any of them - i.e. no tankering allowed ) to recoup what the diversion cost you - to say nothing of the intangible costs of all the peeved pax, the aircraft maybe being late for it's next rotation, etc.
Now whilst it's all very well saying that 'pennies make pounds and that pounds make millionaires', and whilst I'm not advocating "fill the wings and put the trip fuel in the centre"
, but it rather sounds like this particular skipper needs a revisit of 'how to make sensible fuel decissions' and 'CRM'.