Please mark the correct answer so users can find them later on
What I did was order a £10 goody bag but NO extra airtime/credit. This ensured that if I used up my goody bag then I would not be paying extra money for calls etc as there would be no credit to take it from!! Even though I never use up my full allowance from the £10 goody bag I prefer to be overly cautious rather than risk running out of minutes before the goody bag expires. Some may say it's a waste to lose unused minutes or data when your goody bag expires, but it's my personal preference.
The simple way to look at it is to ask yourself whether what you actually used you got for the best possible price.
If you used 250 minutes in a month, those would have cost £25.00 on pay as you go, but as you only paid £10.00 for the goodybag you have saved money. It is irrelevant how many minutes are left over.
If you only used 150 minutes in a month, you would have been slightly better off with the £7.50 goodybag.
If you only use 30 minutes in a month, then 30 minutes at 10p/min is £3.00 and it's likely not worth getting a goodybag at all - unless you also send some texts.