This is the cheapest I've seen Birkenstocks. Limited selection but the Arizona is certainly a popular option.
I got a pair recently and they are super comfortable although they can take a bit of getting used to.
Update: limited sizes available sadly. For those who are concerned about fashion, my stylish sister tells me that these are definitely fashionable as of recently.
I bought these in November when they were the same price. Was a bit worried they'd be counterfeits but they seem to be legit. Such a good price!