This has to do with insurance. You take out an insurance and 'derven' or 'gederfde' has to do with things like 'loss of income' ' loss of profit' when you are unable to use this piece of equipment or material

So what they are trying to say what the cost/expense (price/ day) would be if the equipment is not used, perhaps due to mechanical failure or even adverse weather conditions. You take out insurance to cover those losses.

I am pretty sure that this is it.