I don't know how expensive dextrose is. The gatorade is around $4-$5 depending on where I buy it. I take it on workout days - 3 scoops - usually lasts 3 weeks or so.
In a pinch, you can use glucose tabs from most any drug store. They are pure dextrose, not to mention very tasty. Work well, they are made for diabetics to rapidly rise blood sugars on a dive. I use them all the time. Very cheap, can usually buy a roll for a buck or so. In the diabetic section in any Walgreens for instance, even Super Wal Marts carry them.