I'm using Bottlenose, Dan Loewenherz's "super awesome Python wrapper for the Amazon Product Advertising API". It doesn't parse the XML, so I'm using lxml.objectify:
ACCESS_KEY_ID = "..."
SECRET_KEY = "..."
ASSOC_TAG = "..."
import bottlenose
amazon = bottlenose.Amazon(ACCESS_KEY_ID, SECRET_KEY, ASSOC_TAG)
response=amazon.ItemLookup(ItemId="B0018AFK38", ResponseGroup="OfferSummary")
from lxml import objectify
root = objectify.fromstring(response)
root.Items.Item.OfferSummary.LowestNewPrice.FormattedPrice