the cheapest right now is the alreayd mentioned "diy miner" using avalon chips and assembled by burnin.
however, ASICminer should shortly be auctioning it's so called "honeybee" miners - single asic chips ready to plug in to a USB slot and mine at about 300mhash. the pricing is up in the air though, for now.
i can see some really cool honey bee colony setups now...
oh, agreed. thinking about this makes me want to shop for usb hubs... notthat i could afford a bunch right now... but still!
also, i should point out to the OP that another auction of "block erupter" blades from ASICminer has begun. these are 10 gigahash units, price to be set by the auction. last batch went for 75 btc each, and were delivered in 4 days.
https://bitcointalk.org/index.php?topic=189248