There is NO absolute method for charging for design work. If you really must go by some sort of equation then I would suggest the following:
Technical Expertise = multiplier by 10 (on a scale of 1 to 10) for the hourly rate
Number of Designs = multiplier by 1
Honestly, we charge $500 to several thousands of dollars depending on the client. Why?
You are designing a ROYALTY FREE piece of artwork that will define a company.
Keep in mind, our prices are lower than the larger agencies because we are a smaller operation with lower overhead...
Well designed logos create a sense of trust in a client. Trust in a brand allows you to charge more for your product and or services.
Look at the Nike logo - does Nike produce better shoes? Proabably not but their BRAND demands a premium because we TRUST it.