I would expect to pay sticker for any Yaris model - at least for a while. Don't pay a premium over sticker or accept any mandatory "add ons" from the dealer, though. I was able to pay straight sticker and get good trade value; you might even do better! Edmunds.com is always a good source of pricing info if you have questions.
Since there's not much margin for the dealer to play with on the new Yaris don't be surprised if the dealer is tough on your trade value, if you're trading-in the guzzler.
Hope this helps...