What benefits in bikes like "trek", "cannondale", over "merida" and such? There are more expensive marks like "Trek", "Cannondale", "Specialized" and cheaper ones like "Merida", "Norco" and many others.
What are the benefits of the expensive ones, if say the components like frame material, derailleurs, brakes, BB, cassettes, rims, tires are the same?
You're paying some amount for the branding of the bike, as well as the bike shop to carry the branding and advertising it and supporting the bike after you buy it. A lot of components can be the same - for example, tons of major brand and non-major brand run complete Shimano 105 drivetrains and various other parts (they may have slight model number changes for OEM versions versus retail versions, but essentially the same stuff). 

However, there can be some engineering differences though in the parts which are intrinsically the manufacturer's. This is typically the frame+fork - just because you make two frames which look kinda alike out of the same material doesn't mean that they are the same quality frames, or as well engineered as each other for lightness/strength/stability/etc. [Though I'm not saying that smaller branded products are necessarily worse-ly engineered. ]

Also, typically, the major brand stuff looks a lot nicer with better resale value if you're into that sort of thing. 

It is worth saying that while some places (e.g. BikesDirect) advertise a lot of their frames as "compare to (name brand) at (much higher price)", there are trade-offs in what you get. The frame, for example, may have less nice joins, be a bit heavier, use a lower grade of metal, etc. , so its not exactly the same thing (in some cases, good enough, in other cases, a poor imitation). A lot of people often play the argument that all these bikes are made in similar factories in Taiwan or whatever, but they aren't (usually) coming from the same molds, and a factory can produce a decent spectrum of qualities and types of bikes. [Another thing to note is that places like BD don't have the overhead of a retail shop where  you may get a bike fit or having to hire people to stand in line and what not]

(Unlike in the car world, I don't believe that bicycles are often badge engineered anymore (Schwinn had a bunch in the 70s and 80s for the non-made in USA stuff). But you can get some good bikes from some brands which are cheaper than others.)

As for the price difference, a lot is knowing when to buy - if you buy at the right point in time, the price difference is a lot smaller than usual.