Most generally a pickup truck load is definitely going to be under $50. We have a minimum load fee at our quarry and it's only $15 for that amount, sooo... If you really want a true value you'll need to call a pit that carries that type of sand and ask them.
I pay $275 for a 25 yard dump truck load, so I'm thinking a pickup truck load wouldn't be too expensive. ask around at other horse farms that have arenas, most likely they have good connections on footing, and can recommend good prices/places.