So I was on my way to build a new outpost right? Not anymore! I ran into this while exploring the frontier.
Yeah that was from when temples could spawn in ocean AND river biomes. Someone must have accidentally spawned it pre-fix while walking by a river. Edit: I had one spawn on me in the mountains... it chopped away part of my iron farm.
Not a whole lot, now. Right after the 1.8 drop, they went for 20k, now maybe 1k. Depends on the buyer, a lot.