Like Linda, our family's income has increased significantly since my eldest child was a baby. My DS is now 7.5, and my DH's salary is 2.5 times greater now than when DS was born. I know that not every family is so fortunate, but there's a big difference for us.
We also fall into the category of spending more than what the average MDC homeschooler appears to spend. I'm fully aware that we spend more than we need to, but it's a choice that I've made for my convenience and for our enjoyment. No, I didn't need to buy a fancy curriculum for my kids. But I did because I thought it would be easier for me (all resources in one spot, and I can pick and choose what will work for our family). No, I didn't have to buy books for my kids to read- we could've relied only on library books. But this year I chose to buy readers for my kids, so they'd have books to read that would coordinate with what we're learning about, and I didn't have to search for those books at or reserve them from the library. We still go to the library every week or two (and bring home tons of books), but it's more convenient for us to own some of the books for now.
As for the talk about food, supplies, clothing- for our family it doesn't make a difference whether the kids are going to school or not- we'd still buy the same things. Quite honestly, it'd probably be cheaper for me to pay for school lunch eveyday than to buy the healthier food that we eat everyday for lunch (not to mention the times we go out for lunch, just because we feel like it). Most of the items on any 'school supply' list are things I buy for my kids anyway- crayons, markers, notebooks, lunchboxes, tissues, etc.
The most expensive category seems to be field trips and private classes. Like others, I suspect that we'd spend a similar amount of money on field trips, dance classes, sports activities, theater, etc. regardless of how my kids are being educated. I'd take my kids to the zoo and the children's theater and bowling and sign them up for soccer and dance even if they were going to school.
The only expense we have right now that we wouldn't if my kids were schooled is the cost of classes at our homeschool co-op. It's similar to a once-a-week private school, with classes taught by professionals. My kids take 2-3 classes each, and in total we spend a couple thousand dollars a year. While I don't think it's neccesary, we all enjoy attending, and we can afford it, so why not?
Homeschooling can be done cheaply or expensively- but I think the most important thing is that the family is enjoying learning together.