The $20,000 cost does include the "rebate" for recycling the old one. I don't know that it implies much about the actual battery costs, though. I think the main implication is that Tesla isn't terribly eager to sell battery upgrades, and they'd rather use those batteries to sell new cars, but they'll do it if you make it worth their while. With limited production capacity for the new batteries for the next few months, they don't want a bunch of upgrades getting in the way of sales to new buyers.