what kind of durability should i really expect from my shoes?

So, I know that the rule of thumb for running shoe replacement is "every 300-500 miles."  I've been ramping up my run mileage from the 10-15 mpw category up to the 16-20 mpw category; I usually run 4 days a week.  So I would expect that I would probably need new running shoes every 4-6 months.  I am also on the heavy side for a woman who runs (although I've been steadily losing weight, so right now I weigh about 148 lbs.), which would mean that my shoes would tend to wear down more quickly.

However... I have had my running shoes for about 2 months.  I have only put about 120 miles on them.  They are already showing significant wear in the heel and the forefoot, and the cushioning is shot.  I'm really mad about this, especially since these are not el cheapo no-name running shoes -- they're $130 shoes that I went to the LRS specifically to buy, therefore spending a bit more money than I wanted to in the name of helping the LRS stay open.

Am I justified in my annoyance?  Is it possible that I have a defective pair of shoes?  Should I just HTFU and buy another pair of running shoes, trusting that the amount it cuts into my grocery budget will help me lose more weight so maybe my next pair of shoes will not wear out so fast?