I started collecting plates when I found a box of them at a garage sale for $15. When I got home and checked them out it was a complete set from the 40's to present. Now I have at least 1 plate for every year from Ohio from 1915 except 1919,'43 and '52. '43 and '52 no plates were issued because of the wars.
Before '15 the plates were porcelain and are very pricey. Plates from the '40's to the '70's are anywhere from $20 to $40 a set if in decent shape. Older ones the cost are all over the place which is why I don't have a '19 yet
Jeff