Depends on a few things.How much a mile,Does he pay tolls or do you.Does he want you to pay any of the fuel cost?If he wants you to help pay for fuel,Myself id stay away from it.
As far as percentage pay goes what kind of split is it?Most common seems to be 60/40 split with person getting 60% paying for fuel.If he is offering you anything less then the 60% and you pay for fuel stay away you will have a hard time making a living.
Myself I am payed by the mile but I pay nothing towards the truck.I do alright.I got lucky though I got a good owner that leaves it up to me on what runs I take or dont take.When I want to go home I do.Pays me what I am owed when I am owed it.So far he is happy and so am I.Hope it helps good luck and stay staff.