I pay 100% for my two horses. Feed, vet bills... don't pay a farrier because I learned to trim reasonably well myself and neither have any serious hoof issues to worry about. I don't pay board because I'm lucky enough to live on 10 acres.
I'll be 18 in 2 and a bit weeks, and have been paying for my own horses for some time now. I even include purchase price in that - my gelding I bought on a long term payment plan thanks to his wonderful previous owner (basically a lease-to-own!) and I did buy a filly a bit over a year ago who I recently gave to my mother to make room for my TB who was free. Tack I pay for, and I have two saddles, two bridles, and more bits than I know what to do with.
I am lucky that my gelding, though he isn't cheap, isn't ridiculously expensive to feed either. He costs anywhere from nothing (at present) to $50/week to feed, BULK hay included. In summer they go through a large round bale every two weeks between them and I pay for my share of that (mother has horses as well). He gets an incredible amount of feed when he's in work over summer. The filly I gave away cost next to nothing to feed because she would be fat on a sniff of grass.
My TB is looking set to be expensive. She's a wee bit underweight at the moment so I've upped her feed, and we're smack in the middle of the spring flush when EVERYONE'S horses are fat on grass. Easy keepers are much cheaper to keep, and horses with good feet will save you hundreds!
My gelding, I bought as a finished event horse. Training is not a cost I have to factor in with him. My filly, on the other hand, I will have to have professionally saddle broke, and I have a long road ahead of me to get her ready for that. She's so ear shy that getting a halter on her is tricky at times, and getting a bridle on all but impossible. She also sets back when tied, and rears, and can be difficult to lead - even pushy at times. I'm starting to wonder if it wouldn't be a better idea to have a professional work with her (anywhere from $300 PER WEEK upwards) for some of this groundwork stuff as well.