$40-50 million to start a game from scratch. Think about the hours of work that get put into thinking up what the game is about, creating characters, getting a story together. Then they create concept art for towns, characters, enemies which eventually get toned down and rendered.
Final Fantasy VII has a story. It has its towns. It has its enemies. It has its characters. Already designed. Actually, redesigned in Advent's Children - and apparently the Crystal Tools engine can render AC in real-time. So, that means they don't have to create textures for all the characters. And they can re-use parts of it for Edge and a few other places. And they've got the first couple minutes of the game done as a technical demo.
So, if you note all the things they don't have to do in creating the game, it'll most probably cost half to 2/3rds of a regular AAA title game. And, with one of the most rabid set of fanboys/girls on the planet, it could be one of the most profitable remakes in existence.