Promos are an excellent example of a typical market with supply/demand and pricing.
When they're new, like right now, there's high supply and sellers keep trying to undercut eachother to move it fast for their $20 (198 sellers on BL worldwide, 470 quantity as of this post time, 10/26/17). Not to mention the actual supply from Lego, so many people that wanted one... just got it as a promo from lego. The real market for these are people that missed them, so 6+ months down the line.
Avg sold is $25, a few listings for sub $20 at the moment, many in low 20's, avg list price is $29, though some do move at that list price
If you don't care about moving this asap you can probably get more for it if you wait some months or a year. Freebies generally sell for more once supply goes down, as one might expect. Right now is highest competition.
This is a great promo imo - large in size, 2 figures (an official monochrome), mini lego set stickers are very desirable, white tiles