Both of these rings, regardless of color of location cost 250 Farm Cash. There hasn't been a change in their prices, but if you've yet to pick one up for either of these farms, and would like to rest easy, knowing that your crops (on those specific farms) will never wither again, you can shell out the $45 or more that it will cost you in Farm Cash.
As a suggestion, these rings will be available for the next five days, so if I were you, I'd wait until a Farm Cash promotion (say, a free-gift-with-purchase promotion) launched within the game. At least that way, you'd receive two items for that incredibly high Farm Cash price. Just don't wait too long, or you might miss out on these rings, and be forced to wait until Zynga re-releases them yet again.
Do you have an Unwither Ring on either your home or Lighthouse Cove farm? Do you think the permanent boost is worth such a high price? Sound off in the comments.