It's Pat Frank's Alas Babylon in a Harper Perennial Olive Edition that is priced at $9.74 right now on Amazon.com. There are cheaper alternatives both in paperback and digital, but this is the edition that I found at Dollar Tree this week.
Read about it on Goodreads.
And yes, I got an American classic, in pristine condition with great quality white pages, for a buck. Finding great books at Dollar Tree never gets old!