I'd recommend that you build out a range of packages with different deliverables for clients. Price them based on what you want to make per hour times the amount of time it will take you to create each deliverable (don't forget to build in time you'll spend meeting or communicating with the client). You may want to consider "theming" each package so it helps guide a client to selct the package that would be best for them. Also consider varying the packages both on the end result, as well as the process to get there. For instance, you will want to set the number of rounds of redesigns, as well as the number of options / logos they will get to see at each stage. By clearly showing what a client gets out of the process, this will also help you compete against the companies that are commoditizing logo design like CrowdSPRING.