Keeping Cool comes with a price!

Ouch!  I just got our electric bill for the month of July/August, and it was a lot more than I expected it to be.  Usually, during the winter, it is about fifty dollars/month, and during the summer it is around one hundred twenty-five, but this last month's bill was practically one hundred fifty dollars!  Our house is only just under one thousand sq. ft., so it is not very big, but our A/C is about 28 years old right now(and I think that is the problem).  The person that I had come recharge our A/C this summer said that the unit that we have is a very efficient one for it's time, but I can't help but wonder how much more efficient a newer one would be.