Have you tried converting your colony to offline mode, in statistics bottom of the overview? I'm on an asteroid planet right now for my diamond mine, its currently an offline colony and i can buy aluminium from the black market bazzar for about 4000 plus 2% so around 4100. I don't know if disconnecting from the internet or switching to offline mode will change the price, I haven't gone online with this colony yet.
If you go offline with a colony I don't believe you can take it back online, so I wouldn't recommend that.
Human Colony Charter: pTfi0Opt
Insectoid Charter: EXFzp37s
you can but you loose al you resources