Absolutely - for me the purpose of the game is to keep progressing - up the levels, collecting all the buildings etc, etc. To do this cash is the main thing - you can't build without it and everything needs cash. Especially the higher level town halls, terminals, control centres. Even flights are a way of building up your cash, and passenger yielding buildings are there to support flights.So there is a rhyme & reason on what to get/grab from neighbors?!?!? I just thought you got 1 fuel, 2 passengers and 2 coins (and odd occasional luxury items) depending on what type of building you click on.
On this basis, the digest single cash earners are launches - twofold - firstly the collections to give you the launch items are higher earners, and then the cash when your launch lands is really good along with the other things you get along with a fuel boost. I try and always have a launch in progress and use the forum threads to find who has one going.
So, the bottom line is to earn the most cash, do as many launches as you can, and to do that you need as many launch collections as you can, so the space related items in neighbours cities are the things to go for.
Hope this helps, and each person has their own take of course!