I was receiving an average of 22p per click, but after some SEO (namely buying 100 social bookmark links), it has bombed to only 4p per click!! (CTR is static, but traffic has increased) Why the drop
Well its google's way of ensuring that you don't make much with traffic spikes. Something like adsense sandbox. If your website generators too many visitors and low CTR ( less than 2% ) then adsense will turn your account into a low paying account to maximize their profit instead. It might also be because of spam traffic. Adsense staff might have reviewed your site and found something suspicious. IF that is the case then you might also be banned soon.
Buying a lot of social bookmarking links at once isn't a good idea. This normally alerts Google that something they don't like is going on. Buying bulk traffic is against Google adsense rules. It looks like that your website has been smart priced. What you should have done is applied the changes little by little so it looks as natural as possible. I don't know if the situation can be reversed after you've been smart priced but it is a real blow for your future adsense earnings.
Thanks for all the advice. As an update (yesterday) my adsense clicks returned to a normal price. Perhaps you were correct Ironclad.
you get paid based on what advertisers pay. also depends on your traffic source. maybe the traffic you are getting now is poor quality and from non-=English speaking countries for which you are getting paid low. adsense payout however known to fluctuate to lets not jump to any conclusion right away. give it a few days to see how it looks
ROFL. Sounds like a true business man(smart price).............LOL. Adsense fluctuates based on the advertiser payout, glad that your payout has returned from smart price.