Im new to the forum and just wanted to share my experience on buying a K&N filter. I dont know if this has been covered but I couldnt find it when I searched. I bought the filter not for the small power gain but because I service my car every 6,000 and the K&N is washable so its cheaper in the long run as a Ford one is £13.
I went to http://www.racedynamics.co.uk/store/index.php to buy it and when i typed in my model it came up as £45. I then searched K&N's website for the part number and it was E-2993. I then searched for E-2993 and noticed that the price of the same filter varies by what car you have although the filter is identical. I have a Focus II 2.0 TDCi and my filter was £45, a Ford C-Max 2.0 was only £30.70 and a Ford Focus II 1.6 was £83.80 for the same filter. I guess the C-Max is less common so its cheaper and a lot of people have the Focus II 1.6 so they charge nearly three times as much for the same filter. So if your looking for performance parts check what other cars use the same parts as they may well be cheaper.
If your looking for a K&N filter I hope ive saved you a few quid.