Further to Curtis' explanation of the conversion industry and SLC's observations, although conversion kit manufacturers can potentially make a fortune selling their systems, the reality is that the market just isn't there for EPA or CARB certified aftermarket conversions. Many people looking for certified systems are looking for tax incentives or an HOV sticker. Manufacturers like Technocarb have offered EPA-certified systems for many years but the sales of those systems haven't come close to justifying the cost of obtaining and maintaining the certificates. Curtis suggests that, given the choice between buying an EPA-certified or universal system, consumers will choose the cheaper universal system so universal systems should not be made available. For a conversion system manufacturer, the choice is more like: sell what the market demands or go out of business. Besides, the USA isn't conversion system manufacturers' only market as many manufacturers are based outside of the USA and most sell world-wide.
Using Technocarb's previously EPA-certified LPG system (certificate renewal status still being reviewed) for a 2005 & 2006 Ford F150 5.4L (3V - both 2 & 4WD) as an example, the MSRP is $4142.86. The universal LPG 8-cylinder SVI System on which this certified system is based has an MSRP of $2421.43, a difference of $1428.57. Add a fuel tank (say $800), miscellaneous parts (say $400), a generous amount of installation labor (up to 20 man-hrs @ $90/hr), an EPA-certified conversion would cost about $7150. The non-certified CNG V8 SVIS universal system has an MSRP of $2850.00. Basically, Technocarb takes the cost of certification and spreads it out over a reasonable amount of expected sales. If an EPA certification costs $100,000 and they expect to sell 100 systems, the EPA-certified system could easily be $1000 more than the universal system (including vehicle-specific components and EPA programming). Having an EPA or CARB emissions certificate doesn't necessarily mean the vehicle is any cleaner than one without a certificate. Since the EPA and CARB certificates are only for emissions, a certified system does not necessarily mean that it complies with NFPA 58 or NFPA 52 safety regulations. The universal (non-certified) SVI System and ESIP Package (LPG or CNG), for example, will have lower emissions than what the vehicle would produce on gasoline (when properly calibrated) and will easily meet the safety requirements of NFPA 58 or NFPA 52.
Technocarb offers very competitively priced systems yet there is no market (for all intents and purposes) for the EPA-certified systems they have on the shelf. It doesn't make sense for them to build CARB or EPA-certified systems based on an informal survey. They do their research and provide products that the market demands. Right now, the international market seems to be demanding their EcoDiesel System, which is currently undergoing European certification. If people really want a CNG vehicle, they can start off by buying what is already on the market and there's several on the market already (see NGV America). Interestingly, there are 3 CNG stations in Western NY that are currently selling CNG for $1.2275/GGE and the two local Honda Civic GX dealers don't seem to have any interest in promoting these vehicles.
As for converting an old El Camino, I believe there are still CARB EO systems available (see Can I convert a vehicle in California?) and they would most certainly cost a lot less than $15,000 installed.
I think the real problem is that CNG isn't priced so that people would have a reasonable payback on a conversion. Just being 50¢/GGE cheaper than gasoline doesn't make good economic sense for a conversion and barely justifies the premium for OEM CNG vehicles. It doesn't help that natural gas utilities typically have very reasonable prices for CNG (when they offer CNG) but many don't seem to have any interest in NGV programs.