I've always wondered, do international carriers tanker fuel to/from places with vastly different fuel prices? Example, typically departing for Europe with a lot of extra fuel and returning closer to min fuel?

I know it costs fuel (money) to carry fuel, but I'm sure some accountant did the math on this.

Yes. Where I work the flight planning system figures out the cheapest way to do it using the price at the departure airport and arrival airport. We quite regularly tanker 2000kgs+ into places with expensive fuel.

Most planes landing in the north from the south land at max landing weight, carrying as much fuel as possible. The price delta makes it a no brainer.

Our flight plans show the cost or savings for an extra tonne of fuel. I've never seen a trans atlantic flight with a positive number, you'd burn a third of it getting it there. I've tankered 20 000 kgs but that was a 4 hour leg to an island destination and even then we were saving about $100 a tonne after burning a tonne and a half to bring it there.

We tanker fuel all the time at WJ. Mostly from closer US departure points.(LGA, MCO)

A rough rule of thumb is it costs 4%/hour to tanker fuel.

On a 10 hour flight that's a 40% penalty (400kg to take an extra 1000kg).

At my company whether or not we carry extra will depend on a lot of factors not just the price.

My company flies for multiple clients - each operation has its own unique requirements.