From 89 to 93 is usually 15 to 18 cents difference. 395 into Connecticut is extremely hilly I usually get 29.8 mpg.Where I'm at its like 10 cents more for the next grade above 89 (91 or 93, both at least the Si's recommended octane).
160 miles a day at say approx 32 mpg highway = 5 gallons a day. So 5 gallons times the difference in price (10 cents) = 50 cents. You should get better than 32 mpg though, more like 34 mph (that's what I get while cruising all highway).
Anyway, that's how I got 50 cents or less. I could be wrong about your regions gas prices though, but i have always seen about 10 cents higher for each grade of gas incrementally. But if you're cruising all highway and not getting 32+ mpg then something is wrong, as 34 ish is about what everyone is getting for highway in an Si. The EPA numbers are way off.
Sorry to hear about your situation though, hope things get better for you
And thanks it seems money usually gets tight for everyone at least once in life