[villagetinker]

I see this strayed to hot tubs, so if OP you were considering this in place of a hot tub, IMHO, please talk to a hot tub supplier (we used Spa Kingdom), and see what they may have for your condition. You may be surprised.
With regard to hot tubs, these are very well insulated, I have the heat on in the winter, and OFF in the summer, it actually takes DAYS for the hot tub to cool off. I have seen maybe a $10 to $20 increase (depending on season) for the 1000 gallon hot tub. I do have T&D clean and add chemicals as needed, currently $71/month, I ma start doing this myself.
I do have lower back pain, and I tend to use this 3 times or more per week, for around 1/2 hour, I keep the temp at 100 degrees so I can safely use for this amount of time. Installed cost (3 years ago) was $10,000 which as I understand is below that of a walk in tub.
OP, you may have options, so before you make up your mind look at all of your options.
Hope this helps, feel free to send me a PM and I will be glad to discuss further over the phone.