Could be the cheapest way yet to obtain Catan 3D Expansion Seafarers Cities & Knights. It's current price $250.12 at Amazon US is close to the ATL (RRP $669.99), but there is an option to save 12% on any 2 qualifying items. You could bundle this with the base game Catan 3D Edition $247.40 and get them both for ~$438, or you could find something cheap like Kool-Aid Cherry Drink Mix $3.17. Remember, Seafarers Cities & Knights is an Expansion so you'll need the base to play this. Seafarers is currently rated 8.7 on BGG. Catan 3D was cheaper a little while ago here. Please refer to that deal for more info regarding the base. I didn't spend a long time looking for a second cheap item so there could be other qualifying items that are cheaper.
CATAN: 3D Expansions – Seafarers + Cities & Knights @ BGG
BGO Price History for Seafarers
CATAN: 3D Expansions – Seafarers + Cities & Knights features two expansions to use with CATAN: 3D Edition that bring new — yet familiar — experiences to the table.
In CATAN – Seafarers, you send ships to explore uncharted islands to build new settlements and discover valuable gold.
In CATAN: Cities & Knights, you develop cities and join forces with your neighboring factions to defend Catan against ruthless barbarians!
—description from the publisher
Edit: Was a tiny bit cheaper ($247.40) a few days ago.
I love Catan and have all the expansions, but just the standard version. Would love to get this but just can't justify the price