It's interesting that you say the Harper editions are more plentiful then the Todd school edition, since the Todd edition (Everybody's Shakespeare) tends to be very expensive online. As I understand it, Everybody's Shakespeare contains JULIUS CEASAR, TWELFTH NIGHT, and THE MERCHANT OF VENICE all in one book, and I've seen this book selling for as much as $200-$300 on Amazon.com.
For the Harper edition (Mercury Shakespeare), the three above plays are of course seperated into individual books, with MACBETH added to make it a quartet. Funny thing is, copies of JULIUS, TWELFTH NIGHT, and MERCHANT can all be found on Amazon for only about $10-$15 apiece (I just bought JULIUS for $10). MACBETH however, cannot be found on Amazon, and the copy on ebay linked above sold for $51. Makes me wonder if the MACBETH book isn't considerably more rare then the other three, although I don't really see why it should be.