I can't speak for Greece, but, as a reference, in Italy you can buy a Wind SIM at 15 Euro which includes 10 Euro SIM cost and 5 Euro data traffic. And of course you may add as much more data traffic as you wish. My guess is that other countries will have similar costs, among the jungle of data plans available.


Edit:
Sorry, I forgot the important part: the amt of MB/Euro changes of course depending on what plan you choose. Again just as a reference, with 9 Euro you can buy 1 month of traffic with a top of 2GB data. It means you pay 9 Euros up front and as you start to transfer data you have 1 month of data access and you can download up to 2GB. After a month, you need to pay more. Now, please keep in mind that such plans change continuously, so you may find a better or worse deal. Or, since it is Greece, completely different plans. Mine is just an example to give you a rough idea of what costs are.