Google "gas prices in die hard" you'll see the image that I used. Several of them. I'm too lazy to make this shit up.
Filming took place November 87 - March 88. From the LA Almanac (http://www.laalmanac.com/energy/en12.php), the lowest price during that period was 84 cents, not far off of the 74 cents depicted. As you said, it's possible. These are monthly averages, too, btw, so who knows how it fluctuated week to week or day to day.