Fortunately insulin is dispensed in lots of 25 (ampoules or pens) 3 mls of 100 units insulin. It costs me AUD 46 each time (cost to Govt quoted as $469) with 1 repeat and lasts me about 90 days each script.
A similar problem has happened to me in Australia. The script program used by doctors has a automatic next dispensing date on it of 3 weeks as most medications are dispensed in lots of one month supply.