you arent comparing like for like, you could use a very cheap single hole autogas tank they are built to exactly the same standards however the autogas tank will be airtight and vent outside the car should it need to , If your gas bottle vents it will fill the interior with gas vapour .
A modern autogas "kit" will be a proper fuel injection system that runs a translated injection pulse from the cars original engine management systems and all the original diagnostics will continue to function.
you should compare with a simple single point system which is exactly what you will need to use.
I'm curious what sort of car and what year was it that you were playing with?
the only cost difference is the tank and for safetys sake even get a second hand tank off ebay they fetch nothing as not even the scrap man will take a second hand gas tank.