Quote Originally Posted by Abdula
Because Hidan seems like he really cares about motives, right Bill. All they had to say was you'll get to kill people and maybe fight some strong opponents.
Well, he did say they were "plotting behind his back", and was rather interested in hearing this motive that "lay elsewhere" from the money gathering.

Watching it again, I caught the following:

"Working with us means working for us. If we achieve our goals, yours will be met much faster."

From the sounds of that, I get the feeling that Akatsuki was the one to approached Hidan, and offer their help in exchange Hidan's service. From then on, Hidan's just been following their orders with distaste, but obediently.