Doing the stress tests isn't where the cost would be coming from; the issue is more about the distraction of making sure that compliance is in place.
I'd draw an analogy to having breakfast. If we legally required people to have breakfast every day, that would actually be quite expensive. It'd go from a reflex to something that has to be considered, we have to spend a little bit of time creating evidence that we had breakfast to satisfy the regulator, there'd have to be enforcement where parents check on their kids to make sure they have breakfast, situations where it doesn't make sense to have breakfast (eg, really excessive sleep-ins) now need big reworks to make sure that despite that someone rolls out of bed to have breakfast. The police would roll in and muck up people's life because they skipped breakfast one day due to illness.
It'd be a debacle. It is funny to even imagine what mandatory breakfast would look like if we tried to legally enforce it. It would be expensive, both in dollar figures and hours of lives wasted; no question of it. Just because the act itself is cheap and necessary doesn't make legal compliance cheap and easy. There is a huge gap between the everyday "should do" and the legal "must do".