It doesn't work like this. Your EU guaranteed roaming mobile data package is limited to 2*monthly bill/wholesale-price-per-GB. So in your case that looks like 40/7.7 which is about 5GB. Anything beyond that your telco CAN charge you (but doesn't have to) by usage. I am not sure what that charge can be, but it is per KB spent and is calculate from wholesale price per GB.