I am not sure whether native speakers would naturally interpret "I'll think about it" as a sign of refusal in the following dialogue:
John: Are you available to host our new TV show?
Peter: I've been extremely busy these days. In fact, I am in the middle of a promotional tour.
John: We'd offer you $2000 per episode.
Peter: $2000? Hmm, I'll think about it. Here's my business card. Drop me an email with an outline of the details and my agent will get back to you.
I don't think there is any special use of English here.
"I'll think about it" can be used as an alternative to a direct refusal, but it never means "I refuse". It is a way of not saying either yes or no.
In the example, it seems that Peter is tempted by the offer. He doesn't want to commit, so he responds tentatively. I would understand that Peter is not refusing, but starting to negotiate. He wants John to make an improved offer before he will agree, so he doesn't want to say yes, and he doesn't want to say no. Instead he says "I'm busy", "I'll think about it", "Email my agent". All of which send the message "I will host your show only if you make a very good offer to me"
Mostly ditto James K. Let me add a couple of thoughts.
"I'll think about it" in a context like this can mean (at least) three things:
Literal. The person has heard what you said and his undecided. He wants to think about it before making a decision.
A polite refusal. I don't want to say "no" because that would seem rude or unappreciative. So he pretends that he is considering your suggestion, but really he's already decided no.
As James K says, a bargaining tactics. I do not accept your current offer, but I will consider a better offer.
