It's the same in almost any other hobby... You get what you pay for.
It's like buying a $300 basic computer as opposed to a $600 mid-level computer but putting both in a black case. On the cheap one, you can check email, YouTube, create documents, and maybe even watch a DVD. On the other, you can do some decent gaming, watch an HD movie, and maybe even edit a movie.
Although they both might have black cases, the more expensive one might be more durable, or look nicer. The $300 computer might use cheap components that will burn out in a few years.
Same goes for airsoft; although they are the same model of rifle, some might have features that other doesn't. These features might improve durability, make disassembly easier, make the rifle shoot more realistically, improve gas efficiency, etc.
Since disposable income is something you stressed in your other thread I would not recommend a GBBR. A brand new or well-maintained used AEG would suit you better.
By the way, in the future, you should consolidate your questions of gas guns into one thread.