Amazon offers Prime members the PDP Pelican PlayStation 3 Dual Triggers with Silicone Caps for $1.99 with free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, shipping costs $4.98.) That's the lowest total price we could find by $5. The triggers and caps provide extra grip, comfort, and leverage.
Note, it's currently backordered but can be purchased now for delivery when available. Also, Best Buy via eBay charges the same price, but it may not include the silicone caps.