Your Mom is so right, that pullover is gorgeous and would be a fabulous Christmas present!
I'm really not familliar with their suggested yarn, but to be honest, I substitute yarn more often that not.
I am a fan of Paton's Classic Wool, 100% Merino for sweaters that have a lot of detail because it holds the stitch definition so well. It runs around $7.50 per 100 gm ball, which from my calculation, would cut the amount needed for that pattern in half.
Personally, I find no shame in knitting a $50 sweater over a $150 one...Just means I can knit 3!
And I'm knitting an IK sweater from that yarn as we speak!
No doubt if you look around, you'll find a number or afforable yarns out there worthy of your efforts and your Mom will love!
Good luck with your search and I would love to see the FO!