HamiltonBook.com still has the 1998, 7th ed of the AOU Check-List of North American Birds remaindered at just shy of eight bucks. I paid 50 twenty years ago. So if you ever wanted an AOU check-List it would be a good time to grab one. I have not read what the new AOS will be doing for an 8th edition. There is always talk of online, but the presses keep publishing lots of paper bird books.
Don't buy the 1983 - 6th edition unless you can pick it up for 25 cents. Now the 1957 - 5th edition is a whole different story. It is the last edition which lists subspecies. I think it is starting to get scarce, now in the $20 - $50 range. You just never know what the supply is going to be. It all depends on the thousands of local libraries and when they will be faced with downsizing. They have to get rid of books so they can put in a couple more banks of computers.