It didn't really cost me anything but my leisure time, though. I'm still working on a second one (but I don't feel quite so motivated to finish it in a timely fashion).
If I cared at all about making money off of it, I'd have to hire someone to market it, and then maybe also hire a second person, so that I never have to interact with a marketing/advertising person directly. Making things is fun. Selling things is horrible torture. I pity those startup founders that have to do both, even if only for a short time.