I think that often horse ownership costs are greatly exaggerated. This probably stems from a shared past in places like Europe, and other densely populated areas, where land wasn't readily available. If land isn't readily available it means that land rental costs will be higher, and that feed (which obviously requires land to be grown) will also be more expensive. This means that only certain people will have the disposable income to spend on such a hobby. In this way, wealth and horse ownership have become linked in many people's minds.
Although outside of high density areas horses can be relatively affordable.
My horse probably costs me around $400ish a month with everything included (besides emergency vet). If I lived in an area that had more farmland this could probably drop down to $200 a month. If someone owns their own property then the monthly upkeep could even be lower.
Owning a horse isn't just a flat fee that everyone pays - its not typical of most sports where certain rules and standards are followed. Many owners will have many experiences, some horses might be as cheap as $100 where as others may cost upward of $10,000. The same goes with tack, facilities etc. Some people may "need" an indoor arena, where others are happy with a paddock to ride around in.
So you can see how the costs vary.
I don't pay "board" I pay agistment, which is $220 a month. On top of that I'll have feed which is about $130 a month. Then there is farrier, vet, dentist, which varies but probably equals to about $10 or so a week. Then transport and time (for me to get to the horse and such) which for me is relatively cheap, maybe $20 a week. Then of course other costs, like tack, worming, rugs, feed buckets etc which really gets expensive.