Do you mind sharing which carrier you have now? I've been looking at switching to one of the lower cost carriers, but haven't seen one that supports the iPhone.
Cricket and Ting both support iPhone. If you have an iPhone below 6, you need Ting if it's a T-Mobile iPhone and Cricket if it's an AT&T. I think you can use Ting with the CDMA iPhones. I'm on cricket personally.