I had 2 root canals done last year. They were the result of 2 old broken fillings in the large molars at the back of my mouth. Over the years the fillings had dropped out and left what was basically a hollow tooth, which suddenly became extremely painful one day (I mean really, really, really bloody painful)
The pain ebbed after a few days so I put it off and put it off, eventually signed up to an NHS dentist locally, had a check up and he said straight away 2 root canals will need doing, he did them one at a time, 2 30-40 minute sessions on each (1 uncomfortable session of digging, followed by 1 session of not even slightly uncomfortable filling) followed by a check up at the end. 1 year later, both are fine, I get the odd dull ache once in a blue moon but it's barely noticable. I was in almost constant agony before so the difference is immense.
Cost in the region of £39 I think, but the NHS prices have climbed slightly since then.