For 5$, I'd get one of the common set completers I'm missing, maybe Cherry Jubiliee or Star Hopper, if I could find one for that price.
For 500$, I'd get a Greek Ladybird for my mum. She's in love with her ever since I first showed her a picture. It would be such a lovely surprise. But then, I agree with those who said that they couldn't possibly spend so much money on ponies. If I had the money, I'd give it to my mum to spend. She needs it more than I do.