Doesn't only the cellular version of the iPad have a GPS? If so good for you I guess if you got the money to burn but I don't really see the use for a cellular equipped iPad and a cell phone (assuming the phone is a smart(ish) one). My telco robs me enough with one data plan
Cellular iPads default to being pay as you go for data. Just go into the settings app and buy a few gigs of data when you actually need it.
Some people do have them on a plan... but even then some carriers have "family" plans where you have a single bill for multiple devices.
And if you actually do use a lot of data, then having a dedicated plan for the iPad works out fairly well. Lets do the math:
A 16GB WiFi only iPad is $499 from Apple Australia, but if I used it a lot with tethering I'd have to upgrade my phone's data pack. Adding an extra 3GB will increase my phone bill by $30 per month.
Alternatively, I could buy the same iPad with a 4GB/month data plan over 24 months for [url=https://www.telstra.com.au/tablets/tablets-on-a-plan/ipad-mini-3]$55/month with my carrier[/url] – and $0 upfront, the iPad is "free".
That's $1,219 for a WiFi iPad with 3GB of data per month, or $1,375 for a Cellular iPad with 4GB of data per month. Slightly more expensive but much more convenient and a larger data plan.
So, assuming you actually do use your iPad out of WiFi range frequently it's not crazy to have a cellular plan.