IME, this depends heavily on what facilities you will require, and where you are located.
For example, in my climate, an indoor (or even covered) arena is not a necessity, and my arena only needed to be dressage-sized with sand footing. My horses are on the small size, so 12 x 12 stalls are plenty big. The weather here allows me to have a matted outdoor wash area, rather than an indoor wash stall, and so forth.
I did calculate possible payback on our facilities before taking the plunge, which was nowhere near 30 or 40 years – more like ten, partially because of where we chose to spend money, or not, and partially because I had multiple horses to move home. However, I quickly came to realize just how much I was saving in fuel plus wear and tear on my vehicle (because we found a convenient location, I discovered that I was driving 11,000 miles/year less!!) which I hadn’t considered when making initial calculations.
When combined with the fact that boarding costs meanwhile were steadily rising, these two factors cut years off the time it took to break even on our horse-related improvements. I only have two horses nowadays, but I’m still glad that I have them at home; it remains a very financially sound choice for me, considering current boarding costs in this area, and the distance at which those barns are located.