We have a Coast Spas ‘Zenth’ 6 person one.
Running costs for power are c£1 a day depending on use. Tub gets drained about 3 times a year and refilled (its ‘on’ 365 days a year) again depending on use.
You add chemicals to stop the bacteria and change the filter every couple of weeks (you clean them and reuse them).
Think about where you will put it and access (some houses they have to crane the tub over at a lot of extra cost).
It’s fantastic to be in their looking up at the stars but also thinking about some kind of ‘roof’ so you can use in the rain (even some kind of roll out awning).