I've been wanting to get a bouzouki/octave mandolin for a couple years now, since I started playing mandolin. To sum up, an octave mandolin (and it's similar cousin, the bouzouki) is an instrument tuned the same as a mandolin, but an octave lower. This gives it a fuller sound, and makes it better suited to accompanying solo voice. Basically, it would fill the role of a guitar, but with a bit more of a unique sound, and (for me) be a bit more versatile.
I've tried a few times to build a "cigar box" version of this sort of thing, with mixed success. Decent instruments start around $500, and go on up from there. I can put in a bit of my own money, but as a single dad with responsibilities beyond musical performance, I can't dedicate too much of my "day job" money to this.
Help out if you can, if not, I totally understand. This isn't something I need to survive, but it's something that will help me with my side job as a musician, and potentially increase my earning potential.