That's a great idea! The yarn is Bernat Denim Style. Each skein is 100gms and 196 yds according to the wrapper. I got mine at Joanns on sale and the total came to about $30. I can't remember exactly how many skeins I bought, but I have 2 left. I know I bought 1 extra based on the pattern yardage so I don't know why I have 2, but don't shortchange your yarn..better to have too many. Mine was a small and it's plenty big for me at 5'3". I normally wear a size 8-10 and I have a small frame. I might have even been able to wear the x-small.
Oh and if she can't spend that much she can always give you GC and you can add to it. Make sure your Joanns or whatever store you shop at HAS the yarn though. It would be a bummer to have the GC and then they don't have the yarn! Although you could order it online if necessary.