To calculate the cost of buying Red Snaps, multiply the cost per ounce ($0.25) by the number of ounces (8): $0.25/oz * 8 oz = $<<0.25*8=2>>2.
To calculate the cost of buying Blue Tarts, multiply the cost per ounce ($0.30) by the number of ounces (8): $0.30/oz * 8 oz = $<<0.30*8=2.40>>2.40.
The cost difference between Blue Tarts and Red Snaps is $2.40 - $2 = $<<2.40-2=0.40>>0.40. Answer: \boxed{0.40}.