I have a purchased a hosting that allows me 10 GB of monthly transfer.
Will this 10 GB of transfer be affected if I make some cURL requests, from-a-script-to-a-script on the same server (in this case it's a kind of "local-request")?
No, it shouldn't -- typically traffic accounting is either done at the switch port or network boundary. If the request doesn't leave your machine, no other network equipment will see the packets and they won't be counted.
That being said, local requests via curl are almost always a bad idea. If you're firing off periodic tasks through your webserver, for example, you're doin' it wrong -- you're tying up resources unnecessarily that your webserver should be using to serve customers, and causing yourself potential grief with request timeouts and all sorts of other irritations. There's no reason (apart from laziness) that you can't run your scripts via the command line directly.