That's the $64,000 question, isn't it? Why does anyone become a Sith, or side with the Sith?
The answer needs to have to do with the character's individual personality, not because of external factors like "money" or "power," because those types of motivations just result in interchangeable characters that swiftly become boring.
To side with the Sith is a fast track to destruction. Everybody thinks you're evil and your co-workers are prone to stab you in the back. Not a very pleasant job description, so why do people do it at all? Are they deluded? Insane? Are they actually self-hating and suicidal? Do they just think it's incredibly fun to wield power irresponsibly and they're like addicts, not thinking of the future? Have they convinced themselves that "all those other guys blew it, but I'm special, and I can make this work"? Have they drunk the Kool-Ade and think the dark side is light and vice versa? There are lots of underlying motivations a Sith-allied character could have, but the writing actually needs to communicate that.
Dukat is a great example. He was motivated by power, but not really. His hunger for power was an expression of his unique personality. He was a raving egotist who always thought he could bend whatever situation he was in to his will.
The acting and writing guaranteed that he would be distinct from other power-hungry characters, for instance, Weyoun, whose machinations were an expression of loyalty and reverence. Weyoun did the same sorts of things as Dukat, but for opposite reasons. That's the kind of attention to character that SW needs more of.