I think the Robby robot is ridiculously prices as well, though I did get one for mine. I think its pretty well considered by most TZ owners as must have mod so, demand is high for them.
I think yours looks great and I like adding all the sci-fi related mods to mine (ie robot, rocket ship, flying saucer). If you start making them again, I would be interested if you can get the price point below 200. Even more I think a slightly larger topper version of this would look awesome. I know that that would be a bit more expensive of course (front facing twilight zone cutout) but, I for one may be willing to stretch the budget a bit for it. My TZ is a long term keeper so, I'm fine with investing a little money in it.