"Draw Something" is one of the highest grossing apps on the UK appstore right now. They have a free and paid version. You could take some clues from them.
The great thing about getting money is that you can buy traction with it.
Draw Something (like Pictionary) is in a sweet spot. To begin with it's a game and people play with their friends. They leverage Facebook to find your friends that have the app already. You can still invite people that don't have it yet, but knowing someone already has the app makes it a lot easier to start a game.
They limited some features so they can charge for them. You only get a few colors but can buy more. You get "bombs" that act as hints and can buy more.
One thing they don't do yet is have a way to share your drawing (or animated replay) on Facebook. That would be a big help. I read about a "paint for cats" iPad app (I think the one below) that added a "Share this painting on Facebook" button and it blew up his traffic. Not sure it really applies to illico but it could to a more visual app.
http://itunes.apple.com/us/app/paint-for-cats/id427702410?mt...
Not sure it would work well for this app, but I'm always looking for anything visual or easily shared, especially if your audience are likely heavy Facebook users.
The great thing about getting money is that you can buy traction with it.