When substituting yarns, I suggest you find one that is the same weight. By changing the guage, you end up doing some crazy math to get the dress to turn out the right size. (I like math, but I'm not willing to do that myself.)
Yarndex.com is a great resource in times like this. Patons Divine is a bulky weight fuzzy yarn. To get the same look and warmth (and not have to do crazy math with a different guage), look for another bulky weight fuzzy yarn. Yarndex found 154 options, but only displays 50 at a time when you do an advanced search (the only thing I dislike about Yarndex). I also searched on yarns with the same guage as the Divine (12/inch) and found 213 matches (and could look at all of them!). Knit Picks Wool of the Andes and Quarry yarns are the right size. Red Heart's Cupid line is the right size. Lamb's Pride Bulky (that's a wool/mohair blend). Lion Brand's Suede, Mystery, and Monlight Mohair yarns are the right size. All of these should be reasonably priced yarns.
But, knitting a dress is going to be expensive just about any way you look at it. Sometimes you can find a good price on yarns on Ebay, so keep looking for what works best for you.