I noticed my plunder went down first... After I had 75% of my explored highland converted my plunder went up again
When I was converting my SOS one by one my plunder went up by 18k each volary I put up. With T4 price slash I dropped them all and put up L3 guilds, my last volary I put up my plunder went up 450k(but I dropped an L3 guild not SOS). I'm working on L2 colony, then I'll get the rest volary.
I think it depends on your build, if hansel, it may lower then go up like SOS, if hybrid it should go up assuming your stats are high enough.