Errr.... Huh?
"We really should see somehow where the huge profits generated online go, and whether there is a way to keep some of it in Hungary"
How would putting a tax on data usage "keep it in Hungary"? That would imply that the tax money the people are forced to pay would normally be going to pay for something elsewhere. It's not a sudden shift in the money's destination. If anything, you take money away from what a person can spend on other things, which a majority of the time is already spent in the country (rent, utilities, gas, car payments, insurance, etc).