I bought my first after making the 3k infrastructure jump. I know a lot of people would say that is too early, and it does take a while to get that money (someone helped me out a lot with getting it), but I'm glad I bought it then. In the long run, I think it is better because you can have more wonders than other nations your size, and no matter how big you get or how much you make, there is always the 30 day limit on when you can buy another... My nation is at 5,719.99 infrastructure, I have four wonders (just bought the forth), and I make over 5 million a day after bills...