With modified silencers. Includes switch in lower switch trim to activate the sports sound. In conjunction with Sport Chrono Package/Plus the sports sound can be activated with the sports button. Note: available from approx. 01/2007
I have a much cheaper suggestion with many more choices: A $15 CD with sounds from various sports cars and cranking it up on the audio speakers. Activated with the "play" button and adjusted with the "volume" knob.
A bit more expensive but more realistic retrofit could connect the rpm gauge with a simple electronic synthesizer varying its volume and frequency proportionally to the rpms -- the synthesizer, again, hooked to the stereo. That shouldn't cost more than $200 in components.
Seriously, paying $2k+ for a different sound (especially when the stock sound is among the best in the car industry already) sounds ridiculous. But that's just me.