The Prestone 50/50 is about $8 a gallon. The Prestone 100% is about $8 a gallon. If you just need a bit to top off then the 50/50 might make sense, as it's certainly not much compared to the cost of fuel over the expected life of the fluid. I like to flush with 50/50, and if I need 3 gallons it makes sense to buy the 100%.