How much should I worry about this large crack in the foundation slab?
Sigh. Cheap builders are forever spending thousands on concrete, but unwilling to spend a few hundred on steel to make the concrete properly reenforced. Nobody sees the steel, but the lack of it does show up eventually.

Unlikely to be a major issue, might be a path for water or bugs, so probably sealing it would be best. But consulting an engineer is good for being careful. Unlikely that the house will fall down any time soon (if ever), so probably not too urgent.

Now, if you change to a hundred dollar bill and it gets pulled down into the crack, you have dwarves, and $10 wasn't enough to tempt them... ;^)