How Much Does Horse Care Cost?

  Today I went to a friends barn where she recently adopted a rescue horse. I fell in love with one that is still up for adoption. I found a barn nearby that does pasture boarding for $ 100, and my friend says a bag of feed is about $ 10 and her horse only uses a bag a week. I was wondering if there were any other monthly/weekly costs. I know vet visits are a bit pricey, but I'm just looking to see if I can even afford it right now. I'm willing to give up everything I own for this horse. I'd be a first time horse owner, and luckily I have 2 friends who are willing to help me out every step of the way with training and such. So any information at all would be wonderful.:)
One rule of thumb I've heard (somewhere) - you can expect the care & maintenance for a horse to cost you about as much every 3 years (possibly 4) as the horse cost to buy in the first place. Minimum.''Cheap''horses may cost you quite a bit more in vet bills.
A horse costs thousands. That boarding probably isn't good if it's that cheap. One bag a week seems like too little. Hay costs a lot. Vet bills can be thousands.