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.