Figure up what your truck gets gas mileage wise with a trailer, then divide 900 by that number, multilply that by the price of gas/diesel per gallon. And add that to what your time is worth.
Example: 10 miles/per gallon
900/10= 90 gallons of gas/diesel used.
Gas: 90 x 2.86=$257.4 + what your time is worth=TOTAL CHARGE
Diesel: 90 x 3.08=$277.2+what your time is worth=TOTAL CHARGE
Prices based on US averages as of 4/19/10.
This is how I would figure it up. But since its your neighbor I'd probaly cut them a break on the total charge, helping each other out pays off more than money ever will in the long run. Thats my opinion, not all will agree, but thats what I think is right.