I had to change the idle pully and the tensioner as well. idle pulley was cheap as I used a generic brand but for the tensioner I had to get a genuine nissan one which was about 100 or so from memory. I also changed the water pump too as it makes sense since everything is out at the time. A genuine 1 from nissan was about $150.
Plus all fluids as well it can be a little dearer than that just on parts etc.
Make him change it before you purchase it or bargin him down as its due to be done. Timing Belts are usually cahnged on a time period or 100,000 depends what comes first.