The actual box doesn't cost much to build (TT, 5th, TC)...it's the facilities (fridge, stove, furnace, electricals, etc...)
The facilities are pretty much the same for each. If anything the TC suffers as they may use smaller versions to save space but that results in less economy of scale.
We were looking at them before plans changed a couple years ago (planed to do the PanAmerican Highway). You get a TC because you need the capabilities of a TC. If you are staying on smooth roads and staying in regular campgrounds, there better options. If you need to tow a large boat or want to go into the back country where you need 4x4, TC make a lot of sense (I'm sure others can come up with other use cases).
Since you already have the truck a big part of the cost is already taken care of.
