From my Cessna 172 days, about 17.5 mpg - maybe 140 cruise speed at 8 gph. YMMV.
YMMV - get it?
I'll go away now.
My 172M carries 95 gallons of fuel.
Now I’ll go away. lol
Data not given to you for cost:
Average per gallon Fuel Prices By Region
Updated 12-15-2015
......Jet A 100LL
NC $4.31 $5.22
NE $4.52 $5.40
NW $4.66 $5.76
SC $4.03 $4.86
SE $4.42 $4.91
SW $4.64 $5.45
Cessna 172 use the 100LL fuel.
Then there are the Landing Fees and assorted extra fees that most times are a surprise.