here are some questions to ask yourself because 
ultimately it is your opinion that counts :)

Usually key cels are more vaulable and collected 
because they are drawn by the more experienced 
animators but they aren't always the best cels in 
the sequence. 

Is the background orignal or just a copy *this 
makes a big difference in my book* and does 
affect the cost, but may not matter to you

Does the missing sketch make you want it less?

Do you personally think the differences are worth 
$150?