We don't buy curriculum.
I don't mean we don't buy brand new, or we only shop on eBay, or we look for bargains. I mean, we. just. don't. use. it. I know this is something that will likely change as the Agents grow (the oldest is in third grade) but in the early elementary years it's pretty easy to create a "homemade" course of study.
We use the library. A lot.
Most of our studies use primarily books from the public library. Right now we are averaging about 50-60 books out at any given time (although that number has been known to creep up to 80 or higher). Wanna learn something new? Read about it! Anything your child is interested in learning . . . there's a book about it. Probably 20.
We don't pay for things we can get for free.
There are many wonderful websites for learning at home that charge a subscription fee. We don't use any of them. Why? Because there are just as many (if not more) that offer basically the same services for free.
If your child is a Worksheet Person (like Agent E) there are zillions of options to choose from. A simple Google search will lead you to a never-ending supply of free printables for every subject imaginable.
Also, every Kindle game we have downloaded in the year and a half we've owned the device has been free. Even some of the more expensive learning games eventually have a "free today only" thing going on; you just need to check.