The cost is insane!
EPA certified conversion kit list:
http://www.ngvamerica.org/pdfs/FAQs_...ng_to_NGVs.pdf
Apparently the available EPA and CARB certified kits are not sold retail, just to professional installers. Also, Audi is not in the above list. So, if you live in an area with emmisions OBDII testing, then any non EPA or CARB conversion could be considerd as tampering.
www.mycngkit.com has non epa certified kits,which don't includethe costly CNGtanks. The CNG tanks have to be certified, so that is why they are so expensive. CNG tanks are very high pressure, as compared to the lower pressure LPG tanks. Special high pressure tubing and fittings are an additional cost. An optionalhome CNG refill station is also very expensive.
Beware of any accidentinvolving the high pressure CNG tanks, which could engulf the car in a fireball, and/or take off like a rocket. So, how do you keep that CNG tank inside a hot car trunk in the summer?