Planning is the key. Ad hoc teaching leads to ad hoc spending. Determine the learning goals first, then do some research to identify the kinds of resources available to meet those goals. Lay these activities out on a calendar and coordinate them as far in advance as possible. Make a budget and stick to it. Spending does not necessarily equal quality in homeschooling.
Buy and sell used materials. Homeschoolers are networked, and they gladly sell last year's professionally developed curricula with accompanying books and materials. Homeschooling websites have classified sections for this kind of trade, and don't forget to check social networking sites and your local Craigslist.
Consolidate activities with other homeschoolers. Homeschool cooperatives are springing up in many places, and participants are finding that they can finance them for as little as $500 per student per year. Homeschool co-ops are very flexible. Some combine the full curriculum, and some share time and resources around particular subjects or activities. Large co-ops can significantly reduce the time per parent involved and free parents to seek part-time work to offset living expenses.
Libraries, parks and museums are often free. Integrate formal curricula and books-for-sale with free reading and non-commercialized experience. For older children, consider volunteer opportunities that expose them to new people, ideas and practices. Critically discussing what you and your children watch on the news, or on television programs and films, is a way to teach and encourage critical thinking skills without spending extra money.
Have your child assist in household chores, repairs and maintenance, budget preparation, errands, gardening and other practical activities. Mimetic learning -- that is, learning by imitation -- is how children learn the most in their early years, and it is how most of us learn our social skills. Make your child an apprentice in life instead of merely a student.