Complex sentence: Over the course of the series, mintage declined, and some of the dates are scarce, but with an 1865 mintage over 11 million, a type piece can be inexpensively obtained.
Split into simpler sentences: Over the course of the series mintage declined, and some of the dates are scarce. But, with an 1865 mintage of over eleven million, a type piece can be inexpensively obtained.