I think it's good that your daughter will be given added motivation, but once she gets past GCSEs and chooses to study further (if she does) I would make it absolutely clear to her that it was not anyone else's responsibility but her own to study - even if she fails, leave her to it.
My parents offered me cash to study for my GCSEs. They've always been a bit negative about my abilities, which I don't understand, as I was clever at school, grasped subjects easily, and performed well in exams. My mum decided that I was bound to fail (it was NEVER going to happen) so she offered me £20 for a C, £30 for a B, £40 for an A and £50 for an A*.
Thing is, I was working my arse off anyway, so just continued to do that. I had taken 11 GCSEs with the rest of my peers, and had also chosen to take music, art and drama, which ran after school - so 14 GCSEs in total.
I got 8 A*s, 4 As and 2 Bs .. so £620 (I think), which I didn't want to take, but she put the money into my bank account and said it would help with college (it really did).
I would 100% have got exactly the same grades without the money.
My parents were not rich whatsoever, but I honestly don't think they expected me to even get Cs. I just don't understand why though. As I said, I was clever and I worked hard. If anything I was disappointed with the As and Bs!