For freelance design work, I charge a minimum of $22/per hour. I'm not sure how feasible that is for rendering, given how long complex renders can take (and the fact that they're generally done while you're sleeping, having a coffee, watching movies, etc... or at least they are in my case!), but I do know that in terms of my rates, I'm actually considerably lower than most places in my area, mainly because as an independent contractor working from a home office, I have practically zero overhead. The most important thing is to get good value for your work without making the client raise their eyebrows.