With how popular Zombies are as a casual tribe, Grave Betrayal has found itself in quite a number of mono-Black and Reanimator decks. But being a 7-drop, and the fact you'll ever only want to play one copy, it's been roughly a 50-cent to dollar rare for years. Even foils, which are a favorite of Commander players, have barely passed over $1.50.
In July 2015, however, foil Grave Betrayals began being listed for at least $3 a piece on Ebay and Amazon. Non-foil copies have been in and out of the top 100 best selling Single Cards on Amazon since then, as well. So why the sudden interest? At that time, it had little to do with the supply of Return to Ravnica drying up. It had more to do with speculation around the release of a hot new creature from Magic Origins: Liliana, Heretical Healer.
With the reveal of a powerful new Zombie called Relentless Dead from Shadows Over Innistrad, many Zombie related cards started flying out of inventories everywhere. Not only are Grave Betrayal foils finally starting to show an uptick in price, but non-foil copies are seeing steady growth, as well. Between February and March 2016, Grave Betrayal saw a 30-day increase from $0.68 to $0.86 on TCGPlayer. That doesn't sound like much, but it's a 26 percent jump for a card that sees zero competitive play.
Grave Betrayal foils are now listing for about $6 on Amazon. Non-foils are now selling over $2. This time, however, eBay is showing the opposite trend. You can buy playsets - 4 copies - for around $1 a copy. If you're looking for this card, investing in either non-foils or foils isn't a bad idea at all. The sudden new demand from casual and Commander players will likely push the price much higher.
Grave Betrayal has been a casual hit for a long time and it could easily be a $5 card in non-foil before long. It could be reprinted in the new Innistrad sets, but unlikely. But being a casual hit, these tend to show up in pre-constructed decks such as Commander and Duel Decks. If you can, try to find cheap foils if you need this card. They're always a much safer investment when it comes to these kind of cards.