One way to save money is to purchase the complete collection instead of purchasing each item individually. While this will not save you a ton of money, it will ensure that you get all the items you want and help you to save a little.
Another way to save money on the dolls and the accessories is to search for the ones you want on auction sites or listings sites such as e-Bay and Craigslist. With sites like these you can often find gently used American Girls and accessories with a much lower price tag. One thing that you need to be sure of before purchasing them is either getting pictures of the products if you order them online, or if you are purchasing them locally to fully examine them before you buy. This is important because it most cases, you won't have any recourse if they are broken.
One final suggestion to save money on the accessories for the dolls is to check out alternative solutions for them. In some cases, you can find the same accessories for much cheaper at a hobby store or a doll store. For example, I remember one time visiting a hobby store and finding the exact desk that was sold for the Samantha doll for much cheaper.