Are you a native English speaker? I'm asking just to find out whether take is used in similar sense in English. For example, as I understand - correct me, if I'm wrong,
take in phrase "take it for only" means "buy it for only" as well.
As far as I remember, in French, for example,
prendre also means both "to take" and "to buy". I doubt this is typical for russian (so I disagree with conclusions which @texnic made).
Besides, you question is quite incorrect.
There is a logical flaw in your conclusions.
See, you are saying:
брать or взять (take, grab) is used to actually mean купить or покупать.
Imagine following question:
Why does the verb убить (придушить, изничтожить) means to kill in Russian?
See, the thing is that брать has also meaning "to buy". And that the way it is.
Besides, I'm not sure that
взять actually had such meaning which can be understood in relatively wide context.