It sounds to me like you will be getting the computer after the new rMBP line is released. Is that correct?
If so, when the time comes I would tell your grandmother, There are two computers I am interested in. One is the $1,??? MacBook Air with <specs>, and the other is a much more powerful MacBook Pro with <better specs> that costs $2,???. After thinking about it, I decided that the less-expensive Air is sufficient for my needs, and I would be happy to get one.
That way your grandmother knows what the two options are, she will be impressed by your maturity of decision-making, and she will remember your cost-consciousness. Then she will either get you the Air that you asked for, or she might decide to go above and beyond and surprise you with your dream rMBP (in which case it will have been her decision as to how much of her money to spend.)