Would probably be helpful mentioning where you're located, since prices of bonsai vary depending on location. Also, it's hard placing a definite value on bonsai because it's an art and art is subjective. My teacher bought a big yaupon holly the other week for 900 and thought it was a steal, whereas I wouldn't have paid more than 100 for it.
Looks like a very old nana procumbens. Most likely a tanuki as Srt8madness mentioned. You can probably negotiate for a lower price since its a tanuki, but I personally wouldn't pick it up for 500.