Here is how I am saving money with my iPhone.
I was using an HTC Evo on Sprint and the bill was about $70/month for unlimited everything. (I signed up for the plan when it was about $50/month when my husband had an employee discount...then they stopped offering it to me.)
I just bought a used iPhone 3GS on eBay for about $130 that works on the AT&T network. Yes, I know that there is an iPhone 4 and 4S, but I am not a techy person at all and basically just want the phone for a few fun apps, email, texting and calls. I am now going to purchase the $45/mo pay-as-you-go SIM card from StraightTalk that included unlimited talk, text and internet.
So...my monthly bill will be dropping by $25.00. I should recoup the cost of the phone in several months, then from there, it will just be a savings compared to being on my contract.