Composite sandwich. IS uses a flavor of this, although their skin is a wooden veneer rather than the PVC foam. I think only Placebo offers this exact construction in stock sizes for wakesurf.
I've built a number of them, one example: http://www.wakeworld.com/MB/Discus/messages/87668/636552.html?1227532304
It's far more labor intensive than normal construction, which would jack the price up unless you mold it (and then you need sales volume to justify it), manufacture overseas or BOTH of the aforementioned.