Often, right after I upgrade, I have plans for what structures I want to drop, and I've planned far enough ahead that I'm producing the necessary materials.  Like I'll have my triple glass producing away so I can put down a bunch of farms, and I'm just waiting to build up enough of a stockpile to build.  And because glass is so expensive, I'll see that helpful notification "Need $10,203 to build" (or whatever the number happens to be).  Then that number goes away, and I click to place a building, and I suddenly realize that I just spent $10k when what I meant to do was wait to produce enough glass to not spend any of my cash.

Which is a long-winded lead up to: It would be really nice if the cash cost of building a given structure always showed when I was hovering over locations to drop it.  Maybe with slightly different text, or a different color, when I actually have the cash on hand, but something that lets me make fast decisions about whether I should click now, or wait ten seconds.