Walmart offers the Reach Barrier Air Reflective Water Heater Insulation Kit, model no. 3016, for $22.24. (Amazon has it for the same with free shipping for Prime members.) Opt for in-store pickup to avoid the $5.97 shipping charge. Outside of the mention below, that's the lowest price we could find by $6, although we saw it for $3 less in June. It covers a 40-gallon gas or electric heater and includes temperature-resistant tape.
A close price: Home Depot has it for $22.99 via in-store pickup.