AT&T prepaid is $40/mo for 8GB of data (with the auto-pay discount) So if you use 2GB or more of data, it can be cheaper that Fi (though without the free international roaming)

That sounds like 1.5GB of data/month? You can probably beat that with a MVNO, but not any of the main US carriers. Extreme example being like mint mobile which does bulk purchases (3,6,12months) and goes down as cheap as $15/month for 3GB of LTE.

If you have Comcast home internet, you might be able to use Xfinity Mobile (only BYOP for iPhone atm but they do have a few $100 backs for some phones on their site). It costs $0 for calling/text and $12/gig.

Try Red Pocket, Mint, or H2O. They use ATT or T Mobile. The month to month plans are lower than Fi. For bigger saving, get the longer term plans. Sometimes they have sales on Black Friday for deep discount. People would buy the yearly plans on Black Friday, wait until their current plans expire and activate the new plans.