According to the Morning Call, HBO's president of programming, Casey Bloys, announced the network's extensive plan to keep the series finale spoiler-free: shooting multiple endings to keep the cast, crew, and, most importantly, fans in the dark.
During a speech at Moravian College in Bethlehem, Pennsylvania, Bloys said:
[For the ending of Game of Thrones] they’re going to shoot multiple versions so that nobody really know what happens. You have to do that on a long show. Because when you’re shooting something, people know. So they’re going to shoot multiple versions so that there’s no real definitive answer until the end.
And HBO is not just worried about spoilers from your obnoxious friends on Facebook— in 2017, HBO suffered a major hack that resulted in the leak of outlines of unaired episodes of Game of Thrones. Not only that, but a season 7 episode called "The Spoils of War" was leaked prematurely.
So although more filming= more $$$, HBO is not playing around with the grand finale of Thrones.