I don't know if it's the cheapest, but DH and I use ATT Prepaid and you can bring any unlocked phone. We pay $50/month total for two phones.
I've tried buying used/refurbished phones from Amazon and eBay with mixed results. Make sure if you do this there is a good return policy. We've had better and more consistent luck buying "open box" phones from Best Buy, but they aren't as cheap as older used phones.
If you buy a Tracfone iphone it will be unlocked from Tracfone after 60 days of service if it runs on the Verizon network. Meaning you can keep the phone and switch it to a compatible service provider after you pay for the 60 days of service. That's what I spotted on Reddit the other day anyway. My kids have had iphones with Tracfone service for 5ish years now. I'm getting ready to switch to a different provider with more data allotted per month, so that why I've been doing some research on Reddit.
Who do you have for internet? We use Spectrum and would be able to get a cheap plan through them, so that's what I would probably do.
We also have spectrum. We ended up buying an unlocked preowned phone on Amazon and DH was able to get inexpensive service through his android service provider.