I have to say our experience has been similar - in that the operating costs have not been horrific at all - we leave ours on all year round even, with the only change being shifting the temp lower in Summer (we are in Auckland - so probably less extremes and milder temps). When we went down the path (about 4 years ago now) we found a spa (resold through Alpine Spas) which was made in North America and as such had significantly more insulation everywhere, so what has been mentioned above around just making sure its protected as much as possible will pay dividends in terms of total cost. I might do a double take on what the insulation is like on the bottom and as suggested look at getting something put under ours (as its on a deck) too. 

I had expected the wife and kids to be the biggest users, however as it turns out I use it almost every day/evening through the winter and also in Summer - just love it.

Learning/understanding the water testing saves hassle, time and money - so I encourage you to do that. The chemicals are basic and cheaper when bought in bulk (eg from your local hardware store), and the rest is just testing/math.

Only issue we have had to date has been ozone check valves - just grabbed a bunch from aliexpress in the end and easy enough to swap out if/when they do go. Everything else has just been consumables. Ours is a 5 person spa, which has a moderate amount of jets - not excessive - so probably a middle of the road offering, but has UVC lamp and ozone which evidently help to reduce the amount of chemicals needed to maintain water quality. I did think about going down other options, however the small amount of chemicals we are actually adding, don't result in any chlorine residue to other chemical overload - so have been happy with our decision. 

fwiw I totally rate Alpine Spa's customer service - have recommended to others and would go back there if I needed another without question.