@ria_smith £10 goodybag actually gives you 4GB of data from your 3rd goodybag onwards. However, if you're constantly running out of data on the £10 goodybag, £15 would give you 9GB of data (just £1 more than per your example). If you only run out of data occasionally, you can always start your next goodybag early, so depending on how long before your goodybag expiry date you renew and how often that happens, that could work out more or less the same money wise per, say, calendar year compared to buying a hypothetical data add-on.
