Soldato
Seems odd to offer a pay rise for this, say you lose the contract next year or one of them moves internally to a new role do you give a pay cut? Perhaps one of them has a wife who falls pregnant and can only make 2 trips and someone else in the team agrees to make 6 - do you not want the flexibility to pay the 6 trip guy more?
I'd have thought this sort of thing would be an additional variable part of the pay packet - say additional 'shift payments' each time they make a trip out there - totaling to 3k for 4 trips or whatever you agree with them.
This is probably the best bet. The company i work for, if you have to work out of hours for a job you get paid a flat rate that is quite a bit more than standard pay. You could count this as something similar and pay a different day rate for it to match the pay rise, without giving a pay rise