On ebay they generally run from $50 to $150 depending on condition and what accessories are included.
Like everybody else has said, they're not real useful.
Originally they were designed to work on the A1000 with the Transformer software that emulated an IBM PC/XT.
About the only use for them now is if someone is running a bridgeboard and jacks the 1020 into the floppy port off the bridgeboard.
Curtis
