Ah Gazzo this is how it begins.
Next you will be asking for tips on modding the bike, "Some of the bravest" words on ridersite.
If you are thinking of seriously tuning the engine, ie. Chip, open exsausts and power commander + time on the Dyno then an air box will help
K@N is good for the money. And like andy said re useable.
The filter at a high price on ebay is not going to make the bike much better than a K@N. And tbh very very expensive.
Gazzo Save your money for something else and buy a K@N, (If its induction roar you don't want).
I have a renegade air box on one of the bikes which when I first started with the falco I bought because I thought there were big gains. There isn't.
Maybe a couple of BHP. Although you can "just" tell the difference.
Don't get me wrong if its induction roar you want then the Renegade/Hooked air box is the way to go. Its addictive

I would go with the Renegade/Hooked air box for noise alone and only that over the filter on ebay, although for a new hooked/renegade air box they are similar money to the one on ebay.
These are not K@N but look very similar
http://www.ebay.co.uk/itm/DNA-QUALITY-A ... 2c6856457a
If you want an Hooked air box ring Griff he is very helpful 01827285500 Although the hooked air box can not be bought now he does an equivalent called an Evo air box

I have been told by one of our admins that they are about 100 quid
Tolerance will be our undoing.