Err, I mean that the thrusters will have the same power applied, but will lose thrust the faster you go, so your initial thrust has to be much higher than the lower thrust (using the same power) later in the trip (because KE = 1/2 mv² and you have to pay 4 times the energy for every doubling of velocity).
Whatever mass you're kicking out of the back of the vehicle is accelerated relative to the vehicle. No matter how fast the vehicle is going, if the mass-flow-rate and the delta-v is the same, the thrust produced is the same and the energy cost is the same.