not sure exactly what "didn't really take a liking to her face to face" or "proceeds with her role" means. However, based on my interpretation of those two comments, pay her in full.  

You booked her & you should pay her.

If you do not wish to pay for the entire hour, the don't give her a $100 cancelation fee & apologize.  I require a 50% deposit to book, so this would not happen to me.  If it did I would still expect some compensation for reserving my time.

Generally you pay it all..
Exceptions, Her pictures were not hers but you should leave immediately
She did or said something that was not true that lead you to book but again you should leave immediately.

Always tell her why if you do not pay.  

Simply if no chemistry pay it all.

So did you pay her or short her?

 
If you show up and begin the session you are obligated to pay in full unless she offered money back.  

If you show up but don't begin the date you say "sorry I can't do this" and leave her a portion.  

If you no show you owe nothing.  

If you Show up and see she isn't what you wanted but begin the date you owe her in full.  

 
Basically if you start the date you owe in full.  
If you bail out before it begins you owe nothing but SHOULD offer a cancelation donation.

If she agrees about lack of chemistry, she may cut you a break.  But learn from this and make better choices going forward.

If she doesn't look like the pictures and you don't like that then turn around at the door and leave. Don't pay anything. But once you start things, you've committed and out of luck.

so what is the punch line?

Cause if this isn't a joke then your a fool for asking this questions.  Try going to small claims court and asking judge  Judy!    

In truth, unless she ain't the woman in the pictures and you didn't do research (really?) You get what you deserve. Don't blame her for your failures