Tankering does go on, of course, where the price differental is significant.
Say, for example 38 US cents/US gallon versus $2/US gallon (one of the more pristine examples I have run accross in the last year or so), but many larger airlines buy their fuel in bulk lots, through a fuel distributor (World Fuel in Miami, for example) and pay a set price at most outstations, with only a flowage fee being charged at the outstation.
How large a bulk lot?
It varies, of course, but 10 million gallons is common.
The largest I have seen is 200 million gallons, which was spoken for recently, but not all to one airline.