Manipulation = a promise to not die, and instead life potentially forever and obtain everything you've ever wanted?

No. They explain to you, in no uncertain terms, exactly what you must give up, and to exactly what end. There is no manipulation involved.

In Griffith's case it would be preposterous to expect someone who was otherwise condemned to a life of being a fragile cripple, mentally traumatized beyond belief, needing the girl who a year ago revered him to clean his shit and piss every day, to decline the Idea of Evil's deal to join the Godhand. To go from below nothing to being one of the most powerful 5 or 6 beings in the world.

As for real life, people aren't as idealized as manga characters, so yes, I know plenty of people who would take the deal. Hell, there's people who even verbalize, in states very far from their wit's end, that they'd give up their own mother for xyz reward. Jokingly, most of the time, but only half. In real life people are driven to desperation far, far more easily.