Simplest answer is to get them if you think they look cool, and if you think they’re worth it for the +50 SDB. They both add about +0.5% percent to your aggregate sector defense bonus, and they occasionally do cool animations. They don’t actively behave like a defensive unit. (See the answer to the previous question – if this is what you’re after; pick up some Elite Sector Security.)
As for the price differential, we’ve experimented with a range of pricing scales, but when we first introduced these in Total Domination, we were pricing a lot of the improvements according to how much we thought people would like them. We’ve been amazed at how popular some items were (the flags, for example) when compared other items that we thought would be a hit.
Over time as we’ve added more items, we’ve considered dropping the prices or adjusting the scale, and surprisingly got a lot of pushback from more senior players who got them right when they came out. As we’ve gained a better idea of relative market value of various improvement items, this pricing has leveled out into something more structured. If getting one doesn’t make sense to you, I wouldn’t try and talk you into it. I personally like the RoboPumpkin, but I think I’m alone on that one.