It's up to you, I believe they're worth a tidy bundle, but I'm not 100% sure, as I prefer to keep them and make a tiny little graveyard.
Hey, I keep hoping it'll convince the paparazzi that if my sims will kill the gnomes, then no one is safe! (Sadly, isn't working yet)
One thing you should do is keep an eye on your Mysterious Mr Gnomes. They'll also pop out baby gnomes.