I have never seen the dolls go on sale, but cyber monday has huge discounts on accessories. If you don't need items to be "official" American girl, then try Target for clothes, etc. Their "my generation doll" is the same size, also I have purchased a ton of stuff on ebay, both authentic and generic.
And as a tip to keep in mind, my DD was a huge American Girl fan. We have tons of stuff. Once my son broke her canapy bed for one of her dolls. When I called to get a replacement canapy, which they didn't have the part I needed, they told me to send the bed back and they would ship me a new one, no cost, just shipping. They told me there products are guaranteed. So if you have any problems, call customer service, its been great for me.