I buy all my shavings in bulk. A tandem truck load costs me around 150 bucks w delivery, and it lasts me about a month. Compare that to the $2400 I would spend on bagged shavings in a month for 10 horses (that is if I only used one bag/day) WAY cheaper going bulk!
As for boredom.....I keep old zev and supplement bottles, tie them up on the stall w some baler twine, and the horses love playing w them.
Another thing that is great is soccer balls! I have a soccer ball in some of our horses stalls, and they absolutely love playing w those as well. Just have to make sure they are inflated well so that they can't grab on to them and chew them lol....I just find they are way cheaper then the Jolly balls you can buy and do the same thing.
And for my "beavers" in the barn, I toss a piece of birch tree (cut into a chunk of about a foot long w about a 4" diameter, and they like chewing on that, and that way they don't chew on my stall walls n doors!
Horses are like chips...you can never just have one