I'd say the Bronze Cory (Corydoras aeneus) $1.99-$3.99, the Julii Cory (Corydoras julii), and the Pygmy Cory (Corydoras pygmaeus) $.99-$1.49.
Sometimes you can find Panda Cories cheap. But their price tends to vary. I've seen them for $2.99, and I've seen them for $9.99.