for post workout shakes, you'll use approx 1.5-1.75 scoops of dextrose per scoop of protein powder (this is a very general guideline!) 15lbs is gonna last a really long time.
have you looked locally for dextrose? your best bet is to look in the phone book for a local home brew/beer making shop (they're EVERYWHERE). you can typically get dextrose (a.k.a. corn sugar) for under $1 a pound, often in 5lb bags.
otherwise, yeah, that's the right stuff for post workout carbs. remember, dextrose is hella sweet tasting. it doesn't bother me tho.