We can only get 95 or 98 octane here in UK (as far as I am aware). I tend to mix them up a bit. I have a feeling that I get better mpg on the higher octane, premium, fuel (achieved 39.5mpg on a long run recently), but I haven't tested it properly. As you say, fuel costs just keep going up (there only seems to be one direction of travel, no matter what the crude price!).
Think octane ratings are different in the US. There is a thread on this going back a year or so.
Their 93 is our 97 or something like that..