We use 5% of the load increase per hour. If we intend to tanker 50,000 lbs of fuel, at the end of 10 hours we will have consumed 50% of that above the planned burn without it. In other words, if we'd have planned to use 180,000 lbs of fuel for the flight, we'll have burned 205,000 lbs instead...tankering the extra fuel cost us half of the tankered fuel over a 10 hour flight over and above the fuel burn we'd have had without the extra fuel.
We do look closely at the cost of the fuel at departure and destination and that plays a role in determining what we can tanker. However, our chief criteria for determining how much fuel is the required fuel burn vs. the payload; we certaily won't carry extra fuel at the expense of revenue payload just to tanker cheaper fuel...because we're burn and waste much of the extra fuel. The payload pays the fuel costs...so unless the price difference is grand, or the circumstances of the flight can justify the significant cost increase of tankering the fuel (for safety), we generally don't tanker.