AM/PM water and feed (Owner supplied hay, no need to haul though I order once a month and it is great hay at a good price)
Daily turn out/in (blankets/masks/spray as needed)
Daily stall cleaning with soft white shavings
All day turn-out. Morning until dusk daily.
Your grain fed up to 2 times daily
Your meds, shots, wormers given when needed
so for 300$ they feed the horse hay and grain you pay for, deworm with product you buy, clean the stall and blanket. You may want to factor in a budget like this:
$300 basic fee + $100-$300 hay costs + $40(ish) grain + $70-$100 a month(average) for dewormer/shots/farrier
So you are paying $510- $740 a month, not including routine vet bills, replacing tack, emergency care, etc.
You can work fast food, do cleaning, mow lawns, walk dogs, babysit, odd jobs, such as painting, find a barn that lets you muck stalls, and a number of other jobs. I detailed cars for neighbors, worked hauling boxes for a consignment shop, and did jobs for friends and neighbors like cleaning chicken coups, weeding gardens, etc. I also volunteered to do many jobs at home above my chores, which my parents paid me for.
I bought my first horse at 15, with money I had saved, and paid for my horses all through high school, my parents have never contributed.