In my area there's generally a $0.30 - $0.45/gal difference between 87 and 93 octane. You are right though, it is only a 5 gallon tank, and I haven't let it get under 1/2 tank so far.
I think I can do a good compromise. Since 91 is recommended, and stations in my area carry 87, 89, and 93, and I've been filling up at around 1/2 tank if I alternate between 89 and 93 then I'll end up with 91 in the tank.
Jay2TheRescue 
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