Harbor Homestead (18x18) - 40 Farm Cash (down from 50)
Oceanside Farm (20x20) - 50 Farm Cash (down from 60)
Seaside Farmland (22x22) - 60 Farm Cash (down from 80)
Unfortunately, that seems to be where the discounts stop for now. It could be that different users have different selections of expansions on sale in their own games, but as the Lighthouse Cove currently supports up to the 32x32 size (the Waterfront Land expansion), this 22x22 cutoff is pretty low. Still, by the time you reach that point, you'll have already shelled out 150 Farm Cash, so maybe that's enough (for now).
Again, the game's loading screen [pictured] says that these sale prices are only available for "select users," but I haven't personally come across anyone that doesn't have the option to expand with sale prices. That's not to say this is a lie, but I'd simply make an educated guess that more people have the ability than not. After all - Zynga wants your money, so why not release the sale to more users to entice them to actually spend some? Either way, this sale isn't going to be around forever, so make sure to browse through the Farm Aides section of the store soon to see if the sale prices are available to you.
Do you have the sale prices on Expansions in your store? Will you take advantage of any of the sale prices? Sound off in the comments.