Now is a great time to start a butterfly garden. By planting a few butterfly-attracting flowers together, you can create a space where (hopefully) a variety of butterflies will visit. If you have children, a butterfly garden is a great opportunity to observe nature and a good learning opportunity.
The more frugal way to create a butterfly garden is to use perennial flowers. The upfront cost is a bit higher, but the flowers will return each year bigger and better. The butterfly site contains a list of flowers for attracting butterflies along with other resources (like butterfly species in your state). A butterfly bush, salvia, daisies and marigolds (an annual) are the more common plants that you can find at your local garden center.
Look around in your area for the best-priced source on flowers. Lowes, Home Depot, and Walmart are usually cheaper than the local nursery. You can also order flowers online through places like Michigan Bulb Co although the flowers will arrive as roots/bulbs and will take some time to grow.