All Things Toys via Amazon offers the Kloud City Heat-Sensitive Color-Changing Mug in Light Bulb for $7.99 with free shipping. (It's the second option; the one fulfilled by Amazon is an add-on item with a minimum purchase requirement.) That's the lowest total price we could find by $3. This ceramic mug features a light bulb that changes to yellow when the mug is filled with hot liquid (around 212°F).