340 Demon market value?

So when my wife's uncle heard we were going to the Nats, he asked that I keep an eye out for Demons that were for sale, to get a feel for what people are asking for them.

He had a 71 340 back in the day, butterscotch color, automatic, rallye package. It doesn't have to be a matching numbers 340, just original stock and not heavily modified. A solid, rust free daily driver condition is what he's after.

I saw a few Demons for sale Saturday (the only day I was there), but not many. Especially 71's. In the car corral there was a bright yellow one for sale, auto on the column with bench seat and standard instrument panel/dash. Seller was asking $22k and labled it as daily driver. Another blue '71 had a $15k price tag, but it had a big block in it so did not fit the non-heavily modified criteria. I also recall seeing a dark green '72 for $18k.

So that's not as much as I'd like to have to get an idea of market value. Can anyone here offer some additional info that I can take back to him?

Thanks