"Time"
As a freelance copy editor and proofreader, when a potential client asks me to proof a letter or edit a short story, this involves my time.
A win-win deal is important to me. I proof the writing so the content will look better. The client pays me for my time and efforts so I may pay my bills and eat.
Since freelance work is my only source of income, I ask to be paid for my efforts. It's very important.
I do not ask for a ton of money. I really charge far less than I should be charging.
I have done editing jobs for $1500. before. I did one for $500. I do leave room for the client's budget.
So, the next time a potential client wants to use my skills, know that I will require payment for my time.
I do enjoy proofreading and copy editing, no doubt!
Send me screenplays, manuscripts, short stories, letters, etc.
Cheers!