Verizon is currently running a special 50% off sale on batteries, so the standard battery will run you $20 versus $40. When you go to the website it will show the regular price, but if you add it to your cart, it shows in the cart at the discounted price. Unfortunately, the extended battery no longer appears on the Verizon website, so it appears that they are out of stock. You may luck out and find one in the store. When I got my son the Bionic last week, the store was sold out of the extended battery but they ordered it for me and we received it yesterday ($25 versus the regular price of $50). I had first heard that the promotion was on all the extended batteries (someone at the store at the time we got the Bionic was purchasing an extended battery for the Thunderbolt and got it for the 50% off price as well), but looking at the Verizon site now it looks like the sale is extended to the regular battery also.
I don't think you will be able to find a better price on OEM batteries elsewhere.