A year ago, I'd say "people are selling Pokemon Red/green for over seven hundred dollars on amazon, that's outrageous".
But, tbh, an unopened copy of the original games? Probably still not worth a lot, Heart Gold/Soul silver never opened and with the Pokewalkers still in the box are probably worth more.
I'd say....try anywhere between 30.00s to maybe... 135, since Pokemon Green seems to go for A LOT. As for how much the game itself would be worth, that's a hard question with no definite answer, honestly.