What the costs are like
Cost varies on the amount of horses you own. The plan is to own at least a 500 gallon septic tank per horse.
The price to purchase a bin would be around 400 dollars and building one would cost around 300 dollars. To install one would cost be around 500-1000 dollars, depending on how deep underground you want it. To install it yourself would be completely free except for the tools, which would take a while, but the cheapest installation would be buying a shovel and digging. This would not be an investment in money, but in time.
This would not be completely affordable for everyone, but for farmers this would be a great investment in money and you could help the planet. With the frequent pumping of the tank, the tank would be empty and available for more equine waste and the pumping into the soil would create a nice fertilizer and a great way to get money for farmers.