The grippatank crash tested tank and frames are very simple to assemble, I currently have one waiting in my hallway for me to fit. It's just a case of 4 bolts going through the floor, and the plates that the pump and controllers fit onto have threaded holes making it much simpler to assemble. This might be the reason for the cheaper cost of fitting.
However the fact that they are crash tested has to count for some value as well, a crash tested system won't nessasarily earn any more money however the fact that it won't come through the bulkhead has to be worth it. It's true that you could baffle a wydale tank as much as a grappa tank but the restraint system is a different matter.
Simon.