Walmart offers the Dream On Me Twilight 5" Crib and Toddler Bed Mattress in Blue Print for $29. Opt for in-store pickup to avoid the $4.97 shipping fee. That's the lowest price we could find by $11. It features 80 coil springs, a polyurethane foam wrap, and waterproof cover.

A close price: Amazon has it for $29.99 with free shipping via Prime.