I think grapefruit juice increases blood levels of certain substances or something along this line. This may be the reason why someone reccommended it to take with creatine. But I have read, on another board , that creatine is not a substance that grapefruit juice will improve concentration of. Maybe their is another mechanism to improve creatine supplmentation by using grapefruit juice that someone was addressing, but this an obvious factor about grapefruit juice, so I am guessing that was the reason someone brought it up. I don't think that grapefruit juice has some other miracoulous benefit that will improve creatine supplemenation, unless supplement companies would be using grapefruit juice(or compenents of it) in their product.
Also, from what I read on another board, Citrus juices will not degrade creatine as long as you don't leave it out for days.
I would think buying dextrose is the cheapest carb source and most effective carb to mix with creatine. Grape juice, is also effective, but may cost more, and I don't know if all its carbs come from glucose, I think it has more than glucose.