Should you not have enough by now? You have gained $105 in reward points etc...Therefore you have $405 now which is enough to buy the console :O
You don't gain rewards points off giftcards. You gain 1 point per dollar spent, and for every 250 points you get $5. If I did get rewards for giftcards, off of $100 I would get $2. Trust me, rewards points is the very last way I would even try to get this xbox with lol. The only time I got a big check was from a $2000 purchase I made and even then I only got $40.
If you were talking about me saying I had $300 in the beginning, you have to remember I am using those $300 to buy the gift cards. If I buy an $85 gift card for $55, I now have $330, not $385.