I prefer to charge per-project. But it all depends on the complexity of what you're doing. For a simple design, that wouldn't take much time to put together (no more than an hour or two) I'd start between $75-90 as a starting price, and the more complex they want it the more you charge. If you're just starting out, charging a lower price will probably bring in more customers, just don't get the reputation of being cheap xD