It would cost under £20 including the cable if you know someone who is handy with such work.
BT will charge a vastly larger sum...:-)
Is it for broadband or dialup. You can buy a telephone extension cable for around a fiver to run from the main socket (hide it under the carpet or skirting board) and a data type for broadband could be run in the same way. It cost me £7.59 for a 10 metre BB data cable.
Don't forget that with broadband you will require a splitter (ADSL filter) for the main socket; a phone extension cable could be used from a double adapter.