I have a Vbox and it is damn accurate. See if some local gearhead friends want to pool some money together to share a Vbox. Better than paying out $500 by yourself.
I've never seen a phone app that is accurate. The GPS in most phones has a sampling rate of 1 hertz. That means it takes a reading once every second. The Vbox does it at 10 hertz or every 1/10 of a second.