If your cat is young and healthy and there's just one tooth needing attention then I would say it's going to be somewhere between £150 and £200.
If, however, your beastie is getting on a bit then I'd recommend pre-anaesthetic blood tests to ensure that the op won't push him into kidney or liver failure, which will add about another £40 onto the bill, and if you need more than just one tooth extracting and a scale and polish on the rest then that will add to the bill too.
And of course, if he's having an extraction for an infection then he's likely to need Antirobe or other antibiotics to sort out the abscess or other soft tissue infection too- that's usually another £20 ish.
If he needs to go back for any follow ups you can price them up at 20 quid a throw too, and he may need some painkillers for the post-op days if he's had any major work done which work out at a few pounds a day on top of all of that.
Have I scared you enough?