regular dirt with sand mixed in + a lot of it 2ft min
dont even bother with a Nile unless you have at least a 12x6 or bigger cage
look at this cage roger is building for his niles, now thats huge and lot of space http://repticzone.com/forums/Monitors/messages/1934354.html
Yeah, yard dirt + sand or even plain dirt, check out what kind you have available. If you find your local dirt packs too much you can mix in cypress mulch (avoid the red stuff, cypress is best).