That is a perfect size for using drones to map this out and you might not even have to pay depending on how high you fly.
Doing it using our volume measurement tool will be a lot more accurate than pacing it out!
I would recommend using the Mavic 2 Pro or Phantom 4 Adv (which we have as a refurb on our site for $949). If you purchase from Drones Made Easy you get twice as much free processing so that might be your only cost. I recommend using Map Pilot Lite to collect your data at 80% overlap from about 60 meters.