Did your office use to use Google Hangouts?I've had cellular iPads since 2012. Way more convenient than waiting to connect to iPhone.
I use iPhones for like 2 hours onscreen time a week. Meanwhile, I use the iPad throughout the workday for email, instant messages (office uses Google Chat), etc. while I keep my normal work on the dual monitors on the PC.
Where do you use your iPad most of the time? If it is 99% at home with WiFi, then you get to decide whether its worth spending all that extra cash for minimal occasional use on-the-go or in an airport. It's not just the initial cost difference to purchase the hardware, but also you have to maintain some sort of a plan with a carrier... $$$ may start to add up for very marginal convenience.
$20 a month x 12 = $240 a year. Considering how much these iPads cost, it seems a trivial amount.