Yeah considering its team size, complexity, production values, and development time, it wouldn't surprise me if BOTW is the most expensive game they've made to date.
It is the most expensive game they've made.
There have been very few Nintendo games that were either unprofitable or not sufficiently profitable. The most prominent example of the past decade is Metroid: Other M; while its unit sales look fine, much of that is simply due to retailers offloading the too high initial shipment at bargain prices. This is why the Metroid IP has been on the backburner for a decade despite everpresent demand for new Metroid games.