It's not gonna be good. I get 14 mpg mixed driving about 150 miles a week. On longer trips i average 20 mpg on the interstate. Sometimes 19.
I'm also semi-extensively modded. So other factors come into play. But don't switch because of the price of 93, do it for the benefits. The price is good but the mpgs suck so it'll even out.
EDIT: I should mention that stock i only got about 25 mpg on the hwy. For some reason mine has never been great on gas.