That's not really the pareto principle.  But, while the expected dollar value is the same, I'd go for the $200 w/ greater odds..because I'd still be thrilled to win $200...and not much of a difference of happiness w/ $800.  So, overall, I'd come out ahead w/ the added happiness.

How much am I betting?

This is more of a measure of who is risk averse.  I'd roll the dice and try for the $800.  I'm almost always lucky at the slots, so I'm thinking this will be same. 

80% of "effect" comes from 20% of the "causes" ... 

I'd take the $200 but the 80/20 rule I think is based more around effort and work put in to get something out as opposed to a matter of chance.

$200

Praful Thakkar The 80% chance of winning $200 is more in your favor. In theory, according to the Pareto principle, since 20% causes 80% of the effect, the chance of winning $800 would seem more likely. When it comes to gambling, I would still go with the odds of winning.

I would try for $200.

I'd go for the $200--the odds are with me. (And maybe the force is with me, too!)

 I would choose the $200. The $200 can be invested and earn interest. Besides, the odds are more your favor.

I'd take chance, Praful. $800 for 20%. I am lucky, you know that.

Setunsky nailed this one!

I will go with the $200 risk. Very interesting answers.

I'd go with the 80% because I'm not much of a gambler. I always go with the safe route. Praful which would you choose? :)