Google Fi, at least, doesn't charge any extra for a cell modem: you can get a secondary SIM card and it gets billed exactly the same as using data on your phone.
(That depends on how much it adds to the purchase price though. $100? You bet. $300? Nah.)
I will say that tethering on my phone works in a situation where LTE doesn't work well. However the convenience of not having to worry about connectivity is worth the extra cost for the cellular iPad. I never wanted to be that parent, but it is the path of least friction since my oldest has a lot of acronym mental health issues.