Spring 2019, Samsung U.S. website is showing the battery for the smaller 7000 series at $90 for a 21.6v, 3600mah lithium battery. Samsungparts.com shows the larger battery for the 9000 series at $238 for 21.6v, 4500mah -- increased over a year ago. Experience has been the battery lasts somewhat over 400 cleaning/charging cycles.
Foreign exchange dollar exchange rate changes might be a factor, would have to check.
For comparison the Neato Robotics D3, D5 Botvacs etc. lithium batteries are $64 on Amazon for 14.4v x 4200mah, 60wh vs 78wh for the smaller Powerbot battery. NiMh batteries for older Botvacs are about half the price of lithium batteries, but can be expected to last for only half as many charging cycles.