Amazon offers Prime members the 4M Potato Clock for $5.45 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, it adds $6.11.) That's $9 below the lowest total price we could find, although we saw a similar one for $2 less in February. It does not require batteries nor does it include the potatoes.
Orders placed by December 22 are guaranteed to arrive by Christmas.