pretend to be a client
Or just be honest, and tell them you are researching pricing - most people in this field are only too happy to help.
Personally, I charge by the month. I usually work with people for 6 months or more (I occasionally do 'one-offs' but I like the long term relationship. To get some ideas on what you could be charging, talk to the people you've worked with and ask them how much it was worth to them (if they had no financial restraints at all). You will probably be pleasantly surprised.
And ultimately...what's the figure that feels good to you? On what format? It doesn't matter what everyone else does - you do what works for you!