Cool coin, if it’s what I think it is.
Looks like it’s a 4 dinero coin of Henry III (1390-1406). Kingdom of Castile and Leon (Spanish states). If so, it should be about 22mm, quite thin and weigh around 1.5g. These were struck in ‘billon’… a low grade silver alloy with a lot of copper.
View attachment 2081461
With a bit more detail and a bit less corrosion it would be around $40 or so. In that condition, not so much.
If it's smaller, it might be another denomination, but I don't have the right catalogues to hand at the moment.