That's right. It's not only the battery but as well charge management, overload balancing, fusing and wiring.
I first thought about doing it myself. But the thick cables, massive fuses and other things leave a lot of room for mistakes. I have some skills regarding electronics, but such power installations really require experience, in particular in a car which drives on bumpy roads, has vibrations and a wide operation temperature range.
I would not feel comfortable with such an energy pack not being wired and installed by somebody who knows what he's doing (me, for example...).
I agree that the aux battery is expensive. But if you chose a DIY solution you'll hardly save much with all the high-quality components needed. For a saving of maybe 200€ I'd consider this too risky.