Prices are not decided only by manufacturers, the retail chain will inflate the prices according to selling points even if the intrinsical higher cost is negligible at the basis.
A DC powered J1900 board hence cost the same as an Athlon 5350 + DC powered MB, take an ATX powered combo and the BT 4C combos can be find at 10€ less for the most basic MBs , rest is priced at the same level or even higher than the AM1 + 5350; overall there seems to be a slight premium for the AM1 wich is often justified either by the perfs , the GPU or more simply the I/O capabilities, and this premium is added by the retail stores.
On the technical side of things AMD did set the plateform such that it can work in inner case temps up to 45°C according to their AM1 thermal design guide, hence the choice to add a fan.