in hot weather the average of every car deteriorates. The hot air is thin and carries lesser oxygen. This effects the air to fuel mixture, thus the efficiency reduces.
In case of hybrids in hotter climate the performance of the batteries deteriorates, it's charge retention reduces so it provides lesser input to move the car. so this plus the inefficiency of engine effects the overall average.
You do not need to worry about the batteries these don't fail that easily. In the international market, Toyota warrants the batteries for 300000km or 8years (which ever occurs first). This was for the 2007 2008 models. in 2009 plus models the batteries are even improved. Also the price you mentioned is for the new batteries. I heard that refurbished batteries are also availalbe at around 100000