Pretty sure it costs more than $300 to cremate a horse. More like a minimum of $1000 in most places, not including pick-up. And they generally just cremate the head, hooves, and heart and render the rest. Horse Talk -- When Your Horse Dies
Rendering services aren't available in all areas, but where they are, it really is a good option. It sounds morbid, but the horse doesn't know or care, and it's certainly better for the environment. They generally charge $50 to $200 to pick up the horse.
Whether you can bury on your property depends on your county's regulations, the size of your acreage, the groundwater supply, etc. You can usually find a backhoe for hire in the yellow pages. It's good to have a contact ahead of time so you know you can count on prompt service.
Finally, if your horse passes away naturally or is humanely killed with a single bullet to the head, and it is free of medications or toxins, sometimes zoos, fur farms, houndsmen, or predator sanctuaries will take the meat to feed to their animals.