I happened to come across a discount code when I was looking at the cost for custom printed M&M's again today. You can get a 20% discount if you enter "save20" as your coupon code.
Also, if you're looking into this, this is some information I figured out with a lot of googling:
1. The standard 2"x2"x2" favor box holds ~90 M&M's
2. The 10-pound box has about 100 1.5-2 oz servings
3. There are about 30-35 M&M's in an ounce
4. If you want a full favor box of the 2x2x2 size, you're looking at ~3 oz per box
5. The small bag you can get at the store (usually in the checkout lane) is 1.69 oz
Hope this helps anyone that's looking into M&Ms as their favors!