Your first move is to do some research. Find out why he's interested. If he's looking to build it out or into another service that he offers consider what it would cost him to build it in house and go from there.

If you can't find any info then it gets a bit more complicated. Letting him name a price is a risky play because he can set the "anchor" price low and cause the rest of the talks to revolve around his low price.