We separate homeschool and extracurricular activities. My husband gives me a budget at the beginning of the school year (what he would have paid for uniforms and supplies if DD were going to public school). Lessons (ballet, gymnastics, sports) come from a different fund.
As far as other expenses, we have two other "funds": (1) Grocery fund: I am given $650 a month for food, clothing, books for leisure, etc. for the family (2) The house fund: mortgage, utilities, etc.
Field trips come out of the homeschool fund or when that runs out, my grocery budget. We do not participate in any co-ops.