A bit of a Public Service Announcement:
I was contacted by someone looking for a technical illustration for the front cover of a new edition of a book. I asked the usual questions, timing, sizing, content, and of course budget.
Turns out, there is no budget: "The use of your work would be complimentary with a special acknowledgement given in the Preface of [the book]"
A copy of the book sells for about $175. In 2014, the publisher had revenues of $1.5 billion! And not a cent is set aside for the cover of this book.
In case it's not clear, this is Not. Okay. Never let someone try to convince you that exposure is compensation, or that if you do something free or cheap this one time it may lead to more work down the road. If you're into charity, good on you, but put it towards something more in need than a billion dollar corporation.
And in case you find yourself in a situation this isn't so black-and-white, this should help: http://shouldiworkforfree.com/