The purchase price is only part of the cost of owning a cold plunge. Chiller-based tubs like Plunge or Renu run on standard household power continuously to maintain temperature, which adds a modest but real amount to your electric bill — typically in the range of a small dehumidifier running constantly.
Ice bath style tubs like Ice Barrel have no electricity cost at all, but the ongoing cost shows up as bags of ice, which adds up faster than most people expect if you're plunging daily and don't have a chest freezer to make your own.
Inflatable tubs are the cheapest to buy and to run, since most owners just refill with tap water and a bag or two of ice, but you'll replace the tub itself sooner than a hard-shell unit.