yes it is costing you DATA TIME. but you have unlimited data anyway (otherwise you wouldnt have this phone
) and if you are checking every 5 minutes its better for your battery life that it goes gray rather then turning off.
3 questions
1) do you really need to check your email every 5 minutes?
2) why do you care if it is costing you data time if you have unlimited data
3) you do have unlimited data, RIGHT?