I completely agree - except this is a hobby project, not something serious in the first place. If you're hand-tooling the metal parts for the grenade, CAD designing the parts, and machining them or 3D printing them, it would seem that novelty is the factor.
If you can save 100-150 bucks while increasing the cool factor and having a more fulfilling rewarding experience (like you said, this is Hacker News, after all), why not?
$150 does not seem worth whatever effort would be required to custom code most of the software. Especially for a one-off art piece. I think the goal is to make it functional and aesthetically pleasing, and that's what makes it a fulfilling experience for the artist.
If you can save 100-150 bucks while increasing the cool factor and having a more fulfilling rewarding experience (like you said, this is Hacker News, after all), why not?