Depends on the age of the vehicle. I called them about a year ago to ask and it seems they have a spreadsheet table with age of vehicle and cost to buy out, so give them a call and they should be able to tell you the current going rate. I did post on evowners.com what the rates were when I found out but that's gone now and it's not in archive.org either
Weirdly I was contact completely out of the blue by both phone and email today, by RCI, offering me the battery buyout option. I've never asked for it, and when they wrote to me with updated finance agreement documents ~2 months ago they said the option was not available to me. FWIW it's a 2015 Twiz and the offer price was £750, which is in line with the table that used to be on evowners.