Overstock.com offers the Therma Spa 4-Person Inflatable Portable Hot Tub Spa
, model no. EST5868, for $493.99
with free shipping
. That's the lowest total price we could find by $55. It features 130 air jets, a 900-watt thermostat that heats the water up to 104°F, zippered thermal cover, filtration pump, and two filter cartridges.