Allow integrating of buildings that are not the most expensive

I've found that it is often advantageous to not upgrade the most expensive path at certain times and it would be nice to have a way to do that. I'm not sure if you made it this way because you wanted the game to behave like this, or if it was from a UI standpoint.

The case that I find it is beneficial in is when you can get just one or two of the top building, but many billions of the next building down.