Charging for what I do is always hard to figure out, or at least for me. The talented and sassy Jessica Heshe says it all better than I ever could. Read about it here http://www.jessicahische.is/obsessedwiththeinternet/
And while I have always had a hard time with pricing I have always understood what to do when asked to do spec work. I am ALWAYS insulted and refuse the work. Plz, plz, plz do not do spec work. It hurts us all in all ways. The client gets to ask many people to do free work and then they get to choose whichever artist they want. The rest do not get paid for their work. It completely under values all of our work.
If I had the wall space I would buy Jessica's Should I Work for Free poster. http://shouldiworkforfree.com/