The cheapest option is to just put one of the cone filers on the end of the MAF housing with a bit of silicon tubing and bracket to hold it in place. Will get a lovely roar but won't do anything for power (£100?) - but you'll have to do your own research/fabrication.
The cheapest new option is the Simota copy of the Gruppe M (£300) - didn't have much luck with mine
Next option is the AFE one - cone filter and ally heat shield - with or without the scoop (£350)
Next option is the forum price for the eVenturi one (£550) - came out about 2 months too late for me to try
Next option is the Gruppe-M from CA-Automotive (£650-£700) - I run one of these and can just about hear it over my (Rogue Diablo) exhaust
Final & most expensive option is the Geof Steele, Evolve, CA-Automotive CSL style box with Alpha-N (£1200-£3000) - would love one, but not at those prices
If you want 2nd hand, then you'll just have toi keep your eyes open on here/ebay for someone selling one of the above more cheaply.