David was the official portrait artist of Napoleon Bonaparte, which meant that when it came to putting out political propaganda, David was not only the go-to guy, he was pretty much the only guy. And, as a portrait artist, that meant he had direct contact and connections to Napoleon himself. That's why the portraits of Napoleon are at least worth a ton anyways, thought I'm not so sure of Mary E Dorrel. If anything, David's control of paints and technical dexterity alone would be worth a ton, but combined with his ability to set up a composition chock full of symbolism (propaganda, after all), it'd be more than worth a fair price. Hope this helped!