>>>someone recently told me that the Dongle costs a lot of money over and above the cost of the internet connection
Er . . . yes and . . . er . . .no!
This type of dongle
https://shop.ee.co.uk/dongles/pay-as-you-go-mobile-broadband/huawei-e3533/details
plugs into a USB socket and provides a wireless connection to a
mobile phone network. All data sent and received by your computer has to pass over that network and is charged accordingly (which can get very expensive if you use the internet a lot). Your friend thinks that this is the type of dongle that you've got and that's why he/she told you it will cost you lots of money.
(You'd normally use such a dongle away from home, plugged into a laptop, so that you can get internet access anywhere that there's a mobile phone signal).
However the term 'dongle' is often loosely used to refer to a wireless adapter, such as this one:
http://www.wilko.com/memory-cards-dvds+cds/texet-usb-adapter-wireless-80211n/invt/0288976
That also works as a transmitter/receiver but, instead of connecting to a mobile phone network, it connects to a router (such as the one in your house, provided by TalkTalk). It works as an alternative to plugging in a cable between your router and your computer (so that you can don't have to have your computer near to a phone socket) and won't add anything at all to the normal cost of using your internet.
I strongly suspect that your 'dongle' is actually just such a Wi-Fi adapter and that you've got nothing to worry about regarding any additional costs (because there won't be any!).