I'm not sure I would call these cheap as most of them are at least $24, but they are beautiful bentos all the way from Japan. Some come with carrying case, chopsticks, and a lot of other containers. The shipping is very, very reasonable. I think I paid $2, maybe $3 on shipping last time I ordered from here. Give
Welcome to JBOX.com a go. Here's the link for their bentos:
JBOX.com Search : bento boxes
I attended the Cherry Blossom festival at our local botanical garden and found some bentos very cheap. Some asian food stores carry them, too.