What does she do?
Which response would you have if you were her?
1- Well, it'll depend on how you are behaving. If you are nice, it'll be more. If you keep up this nagging you won't be getting much.
2- I only give presents to kids who are getting a passing grade in Math.
3- Hmm... no idea...
What do you notice about responses #1 and #2? Do you see how the parent is reacting to the nagging behavior? Will it stop the child from asking again an hour later? Probably not. Most likely, they will want to keep trying to get a read on their Present Thermometer -- your mood!
The proper way to respond is simply to NOT respond, use BRAIN DEAD! There's no need to say anything either way. Just go brain dead! Presents are gifts. They should be from the heart and not a result of who nags the most. Explaining that to your child, however, will only lead to them using your words to get back at you so... DON'T. Just say "hmmm" or mumble something really simple and boring like "It's hard not to know." then "I know... " when they try to get in your face about it.
One last comment to those parents who have really persistent kids who still won't stop, use ENERGY DRAIN. Something like "Wow, it really drains my energy when you keep asking me over and over. What can you do to put that energy back in Mommy?". Take a look on my website under "Parent Resources" for an Energy Drain list of ideas if you need them.