In-store only, Walmart offers the Ozark Trail 30-oz. Tumbler in Black for $4.50. (The link is for reference only.) That's tied with our October mention as the best price we've seen. (It's a current low in any color by $8.) This BPA-free tumbler is double-walled and vacuum-sealed.
The dollar store is the most amazing place to find bargains for certain purchases. Yes, even pregnancy tests.