The old adage that they use in the medical profession "when you hear hoofbeats, think horses, not zebras" applies to stamp collecting as well. The low denomination definitives of the late 19th century, early 20th century were printed in the hundreds of millions because in those days everyone mailed a lot of letters, and a good many of them survived. The rare ones that you see selling for huge prices were printed in vastly smaller numbers, a few hundred at most, so you're looking at a ratio of a million to one between the common and the rare, so you can see the unlikelihood of a casual find being one of the rare ones. It's always worth looking, though!

And welcome to the forum, Bert, you'll find us a friendly and knowledgeable group.