If you're not aware, I'm currently building a Power Cube 4. This is an interchangeable power unit that can power a tractor, a brick maker, a lathe, and many other tools. Once I'm done with it, I'll start work on something for it to power. I'm leaning towards a micro-tractor right now.
How much have I spent on this endeavor so far? Has it been worth it? My records say I've spent about $1,365 and 25.5 hours on production so far. A John Deere riding lawnmower costs in this range, but isn't nearly as flexible or durable. As for my time spent, it's been great. I'm learning all sorts of useful skills and applying them.
I'm estimating another 16 hours of work and perhaps $200 more in parts. All told, I'm really happy with the process and experience.