Good news re. the roof rack/box.
After writing my reply I found their website, and so also the cost of the current smaller kayak trailer - so I'm not that surprised at your estimate, as the added complexity & stainless steel are going to make it expensive.
Here's another thought which might increase it's appeal - I'm thinking anyone buying a folding trailer is wanting to store their boat off the trailer (hoist etc), so that frees the hardware to also be used for other things - camping and/or bikes, taking rubbish to the tip etc...
So if the bunks could be unbolted & replaced with other accessories such as a large weather-tight box for example, I could sell my camping trailer which would help justify the expense, and free up even more space in my garage. That would be awesome.
Note I would also want min. 13" wheels on it (like the current photos show), and not 10" as listed in the specifications - I'm fine lifting the boat a little higher, and it would help to avoid grounding out on slip ramp break-overs which happens all the time with small wheels & a long drawbar!
I also like their folding jockey wheel - I have to unbolt mine when hitched, as I can't open the back door with it on.
Anyway, thanks & keep me posted.