http://www.andybudd.com/archives/2006/06/creative_pitches_are_toxic/
That being said, if you REALLY MUST do a creative pitch, you could
consider offering them an "at cost" rate on the proviso that they
can't use the work. If you're charging your full rate, then you could
understand how they would think it fair to use the output (as they've
paid for it in full).
Hence why it's probably best to avoid creative pitches.
Producing a pitch without creative could potentially result in a
designer or agency being at a disadvantage when their competitors
include work that allows a pitch doc to really shine.
If everyone played by the same rules it would be a lot easier to
change client's expectations I guess.