In Alberta, the cost differential between farm and road diesel is about 9 cents per liter. Or about 35 cents per gallon. About 10%. However any farm vehicle can use dyed fuel, even if it's on the road. So my pickup runs purple gas, but the Subaru doesn't. Here you have to get special plates for a farm vehicle, and the only place to get dyed fuel is a card lock, and you have to show your farm registration to get into the program.
The definition of farmer is pretty liberal however. $10,000 gross sales per year averaged over the previous 3 years.