I've run 3 tanks of 87 through my Reserve and 13 tanks of 93, and track my mileage every tank.
Based on my dataset, and going by the computer calc, I get 0.9 MPG higher with 93.
I ran a tank of 93 and then two tanks of 87, then two tanks of 93 and the 3rd (and last) tank of 87, so the 87 was actually a little higher octane. My last 10 tanks of gas have been 93.
You can't draw any meaningful conclusions from your 87 vs. 93 data.
The actual octane level across your first 7 fill-ups (where you switched between octane levels) was...
1 - 91.64, 2 - 87.83, 3 - 87.18, 4 - 91.34, 5 - 92.74, 6 - 87.86, 7 - 91.89
So you never got a true reading of 87 or 93 octane during those fill-ups. Any MPG difference you may have noted is not related to the octane level.
Also, for the last 9 fill-ups of 93 octane you posted in another thread, you had a variance of 4.5 MPG across those fill-ups.
This further underscores the point that you can't draw any conclusions from your tiny sample size of 3 87 octane fill-ups.
The extra MPG narrows the cost difference to about $1.50/tank at today's price spread.
I'm curious how you calculate such a small price difference.
As noted above, your data doesn't support any MPG difference between the octane levels, so you should simply be looking at the straight price difference between 87 and 93.
In my area of the country, there is a ~$0.71 price difference between the two grades.
Your profile says you live in the rural part of central VA. In looking at a random gas station in Lynchburg, the price difference was $0.68.
So if you pump 11.5 gallons per fill-up (which is about your average), then premium would cost an extra $8 or more.