Both my horses are super easy keepers and low maintenance,which makes a huge difference,expense wise,but here's a just a few things I do to cut costs back even more:
I do my own trimming,and both my horses have good feet and go barefoot,so no shoeing costs.
I buy round bales instead of square,much cheaper!
I only buy tack if it's needed,and buy used good quality stuff instead of cheap new stuff.
My horses come amazingly clean with nothing but good ol water,no soap required.
I make my own fly spray,and have chickens,and nesting boxes for the swallows.
I use Neatsfoot oil instead of saddle soap.
I use anything I can find around the house/farm that will work in place of things like scrapers,brushes,lead ropes,ect. (My dad's old drywall trowels work great as scrapers,and heavy duty baler twine braided into a rope with a snap on the end works great as a lead rope)
I buy stuff like brushes,medical supplies,hair ties,towels,ect. at Dollarama. They may not be made specifically for horses,but they work just as well.
I watch for sales,and compare prices.
If my horses don't need it,I don't buy it.
I take good care of what I have so it lasts.