If we don't bill our working time by the hour, software is more or less free to create. Desktop software, that is. But here and now, web-bassed software seems to be all the rage, doesn't it? To accomodate decent amount of concurrent users (tens to thousands) you will need hosting that should cost somewhere around $20 to $300 a month. Maybe somewhere on the $20 a month side for the starters.

Many of the hardware projects, and definitely most of those listed in the original post, can be built with that much worth of parts.

EDIT: But that's just the prototype. After that, you need to build the real product (scale, hardware needs factories for that) and sell it (again easier with software). But this doesn't mean we should only do software because it's easier; otherwise we would be like the proverbial drunkard who kept seeking for his money under the lamppost, because it's brighter over there.

Although these hardware kits are open source, it takes quite a bit more money to try them out. I have tons of open source software apps running on my system, but can only really afford to try out several open source hardware platforms.