iPhones are in such high demand that you will rarely see them going on sale. This makes it hard to purchase them on a tight budget. If you really want one, then you will simply have to be strategic about your purchase.
One thing that you can do is to time your purchase right. Get a recent model after a new generation is announced or released. This will surely shift people’s attention on the latest offering which will force the company to drop the price on the previous generation models.
You could even trade in your current phone when you buy the new one. With an iPhone buy back program, you can get either cash or credit for your unit. This can be used to slash the price for your order.
Check the buy back prices for various models to see how much you can get today.