Buy your blades from ebay, 100x astra premium are (or were a few months back) less than $15, and if you bought larger quantities they got cheaper, so even if you only get 2 shaves that is less than 7.5 cents per shave.
Out of interest, how do you know which side of the blade you have already used, i.e. are you using the same edge a few times, and not both edges?