Amazon offers Prime members the Stalwart Ultimate Comfort 16 sq. ft. of 24" x 24" Interlocking Foam Flooring Tiles
in Black, model no. 75-6402, for $20.91 with free shipping. (Non-members can spend $25 to qualify for free shipping; otherwise shipping adds $7.28.) That's the lowest total price we could find by a buck, although we saw a similar set of tiles for $2 less in June. These 2-foot x 2-foot foam-rubber floor tiles are water-resistant and slip-resistant.
Update: As one of its daily deals, THAT Daily Deal offers a 20 sq. ft. 6-Piece Interlocking Cushion Floor Mat Set for $13 plus $3.99 for shipping.