I was in Khobar yesterday and passed by the street with all the hire trucks. I asked how much to Bahrain without specifying the cargo and was quoted a price of 700 Saudi Riyals which is about 105 Pounds.
However, this being the Middle East, you're expected to haggle over prices. In fact, they are rather disappointed if you DON'T haggle.
My guess is that you could haggle the price down to 300 or 400 Riyals. There were a lot of idle trucks sitting around so the competition amongst the drivers would be intense.
The motorcyclist encounters the road as the writer encounters the page.