There are two main reasons why Turntimber Symbiosis is currently priced where it is. First, it's a great addition to commander decks that need to run out big creatures and easily tap for 7 mana. Second, it's a modal double-faced card, which enables Mono Green players in standard to keep mana flood under control.
It's hard to determine how well this card will do, but there is substantial amount of support for these types of cards, so it could go either way. As usual, the best strategy with brand new cards is to just wait and see how the market moves.