I'm having a hard time finding out which node it's on and the die area. The S5 is essentially a re-badged S4 which as far as I can tell is made on standard TSMC 7nm. If I were to hazard a guess, I'd say the BOM is dominated by the casing and speaker hardware. That chip has been in production for about 2 years now.

Display + restrictions on internals/construction due to size constraints will make up a large part of that cost as well. I'd imagine that the chip itself is a pretty small percentage of the price of an Apple Watch.