It's £25 for an average telephone extension socket, so they won't charge you much more than that.
It all depends on how cheaply you can get the FT100/CT100 or FT125/CT125, and if it'd be cheaper to do it yourself.
Do make sure that you know what you're doing before you make a start, though - it's not rocket science, but you can make a balls-up of it if you haven't done it before.
