There Manufacturer's Sticker, like on a Car or a refrigerator:
The House is worth what the Seller thinks it's worth, and what the Buyer is willing to pay.
If the Seller has a ridiculous inflated value, maybe all you can do is walk away.
You could have your Realtor include a CMA with the OFFER, to show her the error.
Also, your Realtor could print out the Tax Record which shows the sqft.