No offense to pbs911, but I have never seen a "good condition" 993 for anywhere near $21K. I am not saying pbs911 is incorrect, just giving you my experience. My experience also tells me that you may be lucky to find one in in the high $20's, which will be snapped up in a big hurry. Low $30's is a more realistic starting point for a good condition 993.
I live by the old saying,
"you get what you pay for". 