If you have prayerfully set goals for your child before going to the Conference or Book Fair then the Lord can direct you to the workshops you need to attend or the books that you need to buy. You can see books and keep them in mind for next year maybe. With me, I was always too poor to buy much anyway. I looked for what I needed in order to achieve my goal. That helped immensely and made the conference more profitable.
Scripture teaches contentment also.
Carefully consider the alternatives in your situation. Of course I had to have time to do this. Maybe you don't. If you have teens in your house, sometimes you can get them to help you. This keeps them from being bored in the summer, gives you the help you need and gives your 3rd grader a neat, new item to help with multiplication facts for the next homeschool year that cost next to nothing.