How about a "building scrap" special item that you get from destroying buildings that you can later use on missions?
Some examples:

Piles of Trash: can be done 5 times
The immense number of people under your rule put out a lot of garbage that not even the recyclotrons can handle. However, we have developed a way to fuse the entire mass into a self-sustaining planet. Provide the geological team with enough scrap and we can forge it.
Cost: building scrap (and a lot of it)
Reward: Quantum Compactor (when used it creates an average sized "junksphere" planet with no research, very abundant artifact, and rich mining.

Mechanical Harvest: can be done 3 times
Our mining constructions sometimes fall into disarray. A thorough replacement procedure will help maximize our ability to gather minerals. Start updating your facilities and we can get to work.
Cost: building scrap
Reward: Equipment Restoration Upgrade (when used on a non-gas planet it increases the mining production level by 3)

Gift to the Scarl: can be repeated
A race of rust-eating avians called the Scarl are the most productive miners in the galaxy. As long as we create proper accommodations for their unique way of life they are willing to work on our planets. It will take a lot to please them, but thankfully our leftover construction materials are perfect sustenance for them.
Cost: building scrap
Reward: Scarl Hospitality Center (building, size 1, 1 per planet, provides 100 population and +2 to mining)

the pile of trash could be better. sounds like a crap planet if you ask me, whilst the other two options sound pretty good. +3 to mining is better than a toxic purifier, and the buildings are always fun to get, especially ones that have multiple values. Maybe the junk sphere can get rich-3x megarich artifact considering the entire planet is made up of other peoples 'artifacts'. the extremely abundant-very rich mining would be good considering all the possible recyclable materials.

Quite possibly is made of actual crap!