On Ebay, I've seen a few of those KClO4 generators selling for around $150. They make claims of producing up to 1 pound of KClO4 a day, which I'm sure is complete bullshit. Do these things do ANYTHING at all, or are they snake oil?

Cheaper and more rewarding/satisfying to make your own.

What kind of yield can be achieved with those machines? What would be the yield/cost versus a homemade system?

I doubt one of those would actually work, if it does there is no way to tell cause they don't tell you exactly what your getting. Maybe a 1lb. theoretical yield but no cell is going to operate at 100% efficiency. There is a thread probably still on the front page maybe second on Dd cell. Here are three links to get you started and show you what your getting into.

http://www.geocities.com/CapeCanaver.../chlorate.html
http://www.vk2zay.net/article.php/63
http://www.wfvisser.dds.nl/EN/kclox_EN.html

yeaaahhh...bullshit They are known as chlorate cells. There is also no way you can make 1lb a day unless you have a very large power supply. Trust me, I made a few.