Hertz and Avis want about $300/day for a Toyota Hilux in Argentina without driver. I know the rates are similar in Chile. I definitely need a 4x4.

I know someone in Argentina(from previous trips) who has a similar 4x4 that's 3 years old and he has agreed to drive me on a daily basis for a set fee + vehicle rate. How much should I offer them per day for the use of their vehicle? He says whatever I think his 4x4 is worth is what I should pay him in addition to the daily driving fee.