Amazon offers the ASUS 802.11n Wireless PCI Express Adapter, model no. PCE-N10, for $9.99. This $10 mail-in rebate drops it to $0. Plus, Prime members receive free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, it adds $8.22.) That's the lowest total price we could find by $23. It features WEP, WPA, and WPA2 encryption. Rebate expires November 30.
Although it's currently out of stock, it can be ordered at this price and will ship when available.