Mike,
I've used a 25 gallon tow behind sprayer from Northern for our riding ring. Think of the extra 10 gallons as 62% more capacity. Means you only have to go back and fill the tank twice instead of three times. $50 bucks doesn't sound so bad then. Depending on the size of your ring and what you are spraying, one 25 gallon load may not be enough, 16 gallon most likely isn't enough.
Jeff