I was expanding my arena, so I entered the numbers and asked for an estimate. I didn't have enough money but wasn't told that, and clicked on the button to expand. I then got the message:
"You don't have enough money to build such a large arena."
So I reduced the amount of my expansion, and clicked again. This time the expansion was started, but the page still says:
"You don't have enough money to build such a large arena."
When you make an expansion, the entire page refreshes and you are positioned at the top of the page, so the first thing you read is the now false error message.
+Suggestion: When you are given an estimate, and don't have enough money, you should be told that, and not have a button to try to expand.