For the first wave of countries where the iPhone 13 series will be available, the phones can be pre-ordered from the 17th (Friday) and should start shipping on September 24. Some countries (e.g. Thailand, Malaysia) will get them later with pre-orders starting on October 1 and shipping a week later.
Keep in mind that some of the older iPhones are still available. The iPhone 12 Pro duo is gone, but the 12 and 12 mini are still available and in the US they start at $700 and $600, respectively. The iPhone 11 is also still around, starting at $670, even the SE remains on sale.