Motobecanes are a really good deal, but they make you appreciate what a LBS does to make a sale. Assembling, tuning, fitting, going back after a month for some more tuning, endless advice, and most of all helping you get the right type and size bike.
I just got a Motobecane and couldn't be happier with it, but I'm out of work right now and have far more time than money. I think they are a good option for those who would prefer to spend time over money, reasonably mechanically adept, and not too proud to pay a LBS for a tuneup if necessary.
But the cheapest option would probably be Craigslist, if you get lucky and the right bike shows up.