You asume that wide Empries will have exatly 0 valid ascension perks in the slots 1 through 4. And that simply does not add up.I think the idea that only small empires* can or will use megastructures is a bit daft. Everyone is going to get some manner of ascension perks I'm assuming.
If you can not afford the ascension perks (you need 2 with 4 slots), then the mineral costs do not mater. That is the point of Traditions, Unity and Ascension perks: Economy does not mater. That rare selection you do does.