The new Lillie 397/SM-P promo that was announced 2 weeks ago, is selling in Japan around the price of $935 - $1400.
Lillie is the most wanted out of the three Full Art promo's you can obtain. The cards you can get in these packs are Acerola 395/SM-P, Guzma 396/SM-P and Lillie 397/SM-P.
You will notice Lillie is crying in the illustration (as a final goodbye), this might be a reference to the upcoming turn 1 supporter rule in which you can't play Supporter cards anymore if you go first turn 1. That makes Lillie as a card not as good anymore in the future.
How to obtain it:
You can only obtain these cards in special packs given out during special Japanese Expanded (Extra Battle Day) tournaments until the end of November 2019.
To make it even more difficult to obtain these in Japan, participation for these events is limited to one per person per month. Also to add another level of difficulty, you must win a lottery beforehand to get to participate in these events.
Apparently people can hold a flashlight against the sealed pack so they can see what card is in there, that will determine it's price. As proof, people take a picture showing it has a guarantee of Lillie. Some Japanese people are even trying to ask sellers beforehand if they can hold a flashlight on the pack before they want to know what card is in there.
Unsearched packs or packs that has no Lillie in there go for far less, around $600. That is what Acerola is going for. Guzma is the cheapest promo out of the 3. I have seen prices for Guzma selling on auction around $200 to $375. Right now prices are very inconsistent and there is a certain level of hype around these promo cards / packs because they are just released.
Why is Lillie so popular in Japan?
Lillie is considered a very popular character in Japan, she fits into the "Kawaii culture" of Japan. Her SM4+ GX Battle Boost (Ultra Prism for us) Lillie SR version is selling consistently for around $150 today in Japan. Naoki Saito is also a very popular artist in Japan. This card is so popular, she even got her own official cardboard display stand during Japanese Champions League events.
Last year in the Summer of 2018, that card skyrocketed to from $80 to $450 because it was suddenly a playable card and many new fans came. Even the Sun & Moon Base Set version of Lillie SR was around $150 (if i remember correctly). Many Japanese players / collectors wanted this card but SM4+ GX Battle Boost was long out of print, this set was the equivalent of SM8b GX Ultra Shiny and SM12a Tag All Stars.
During the same period, GX Starter Decks were released and that contributed to many new fans in Japan and a long period of no availability of packs (to the point of Japanese card shops reverse importing Japanese boosterboxes from overseas sellers). Even the English version during the same period, skyrocketed from $30 to $80 and is still worth around $50-60 today. And the English Sun & Moon Base set version went from $10 to $20.
And to end it, due to her popularity at the launch of Sun & Moon, her first Deck Sleeves during the Sun & Moon TCG release were instantly sold out within 3 minutes and scalpers resold it for $30 on auction. Loose sleeves were spilled on the ground everywhere at some Pokémon Centers. There was no purchase limit, since then Pokémon had learned from their mistake.