Comparing 91 to 95 wont work. That will be like comparing 95 to 99, or 87 to 91. The leap is quite big in both price and performance (if the car is made to run on a certain octane, which European cars are). Still though, comparing 95 to 83-87 is quite a stretch.
93 to 95, quite possibly is similar enough. How much does that cost right now? I know the premium is usually 91, though, so looking at pictures of gas stations wont really help. (Also, why the hell are they advertising in names and not the octane? Car books tell you to use a certain octane, and not having that information is annoying. Using the wrong fuel can seriously damage the reliability of certain engines \
Edit: God the 500 is sexy. Such a brilliant car in every possible way, and even has a brilliant sports version