Ipad Options

The Cellular option allows the iPad to have access to the cellular network and access the internet over it, when Wi-Fi is not available.

It can connect to the internet like a cell phone. Do note it cannot place calls and send SMS messages on its own however. It requires a connected iPhone for that.

Also note, that the cellular service on an iPad will require a contract with a Cellular service e provider and a monthly fee for it.

The cost will largely be determined by the monthly data plan that you choose - more data usually equates to higher cost.

Shop around between carriers; the Cellular Carrier that you have chosen for your iPhone may not be the best option for iPad. EE (owned by BT) offer some great options.

Consider also that you may be able to use hotspot tethering, over WiFi, from from your iPhone:

How to set up a Personal Hotspot on your iPhone or iPad - Apple Support

Carefully consider the apparent economy of selecting a WiFi Only model of any iPad. Only the WiFi+Cellular models include GPS functionality - the WiFi Only models do not. 

We frequently see complaints and dissatisfaction from users who subsequently learn of this limitation after purchase; not only does this fundamental difference affect the reliability and accuracy of Location Services, it can also influence use of some Apps - such as mapping.

Selecting a WiFi+Cellular model of iPad doesn’t lock you in to using Cellular connectivity, but in addition to providing GPS functions, does provide the flexibility of using Cellular connectivity if your needs later require this functionality.

That's entirely down to the provider. As mentioned there is no pre-set price.

You can see BT's prices here: https://www.bt.com/mobile/data-sim/