It all depends on the market. It's always changing. Short letter domain names like 3 letter domain names are always in high demand and easy to flip for a little more. But everything else depends on the market.
Before starting flipping, keep an eye on the domains that sell, and the requested prices. That will give you an idea of the market.
Also stay away from domains with high prices - you will get burned! Good business is made by flipping low-cost domains, as it is much easier that somebody shells out $60 than $6000...