Roaming calculations

Hi, looking at taking out a new plan but to decide which one I need to understand how much roaming data is available. I have found a calculator into which you put the ‘plan cost’ and it gives you an amount of data but what is the plan cost - is it the monthly amount you sign up for or is that split down somewhere between device and plan (as both O2 and Virgin do)? it says you can see it on the app but as i don’t yet have a contract I dont have an app! thanks