Before the C14 reprint, Twilight Shepherd actually was a $2.50-$3 card due to her usefulness in Commander. In particular, she's seen a lot of play in Kaalia of the Vast and Avacyn, Angel of Hope Commander decks. Her ability to return cards to your hand that died that turn is very useful, especially for only six mana. Having Persist means that she can do it once again, as well. Plus a 5/5 vigilant flying body is definitely nothing to laugh at. This is a solid Angel, albeit best at home in Commander.
But being reprinted in a popular Commander deck, plus the second (albeit limited) reprint in the Duel Decks Anthology really hurt her value. However, the Shadowmoor version is still sought after due to being the rarest version. Still, unless you really want the original printing, sending your Shadowmoor copies off for $1 and rebuying the C14 ones at $0.50 seems like a solid plan. If you're going to invest in Twilight Shepherd, the cheaper Duel Deck Anthology and Commander 2014 versions seem to be the way to go.
Typically, cards like this that do have real demand, but only a limited audience, tend to see all of their printings eventually find some sort of price equilibrium. So if the Shadowmoor copies eventually crawl back up towards $3, then expect the cheaper copies to steadily follow suit.