It all depends on how perfect the nebari is.
For a large yaupon holly, it has to be really good, close to perfection. Which means that there should be no gaps without surface roots, and the angle at which it touches the soil should be near tangential. So, when taking pictures, it's very important that you uncover the WHOLE NEBARI (including the top part of the roots), otherwise the viewer has no idea how good it is.
If the quality is there, I imagine that for a 5+ inch nebari and an interesting trunk, you could ask at least $150, or more. Not sure about $500, but for a really great, monster trunk, and a 6"-7" nebari, $300-400 is imaginable. Just for comparison, a trident of the same size would be about double the price.
But if the nebari has flaws, the price would come down to earth very quickly.
P.S.: I don't think that the dwarf variety would grow to the sizes mentioned above, in a conceivable time frame. So, if it is that big, it must be the regular variety.
P.P.S.: Yaupon hollies are one of the best bonsai material that one can imagine. They are fun and very forgiving, not too many species can surpass them in any respect.