Expect a rate of between $120 and $200 to commission from a high quality, reliable freelancer with a portfolio and references. 50%-ish more from a mid-tier agency. You can also find freelancers who will do it for a lower rate, $50-$100. But then you up your risk. You should probably expect to commission a few trial pieces to find your guy/gal and inherently accept the risk quality could go up and down depending on their enthusiasm/current workload. And that, if they are good, you are getting them at still 'starter' rates that could well go up as they establish themselves more.