Many players that have been launching for awhile have a lot of these buildings sitting in storage. I haven't checked on Alien legacy because I've never have more than 6, but the good news for the other two buildings is that if you sell the ones you've built in your city you'll get 1500 coins for a rocket garden and 2500 coins for a space travel monument. When you put another in its' place, the instant build cost is only 500 coins. Clear your warehouse and collect coins at the same time. Sweet!
Be smart about it, though. Only ever do it just after you've collected your drops. At 24hrs a pop, you don't want to waste a day
Be smart about it, though. Only ever do it just after you've collected your drops. At 24hrs a pop, you don't want to waste a day