Septic fields, am I being ripped off?

I know.. He had initially said, "Don't quote me on this because I haven't planned your system out yet, but I would say roughly $8K to $12K max, I have only once in 30 years done a system that cost over $20K, and that was on the ocean's edge, on a cliff that was entirely rock, and I had to encase it in concrete."
Then he calls and says, "OK, I've come up with a price of $15K" .. what?

On the next bidding contractor, tell them you have 10K bid and you thought that was way to high.