If you needed the 200 minutes, you can only use a code one time on a phone. So you wouldn't have been able to buy multiple smaller cards and use the same code multiple times you used for his card to get those particular bonus minutes.
My rule of thumb is I try to find a code that give me 50% or more of the minutes on the card as a bonus (100 minutes for a 200 min card, etc). If I cna do that, I don't worry so much about the math.
Occasionally I will find a great one like 60 bonus minutes for a 60 minute card.