The question I would ask is whether or not $300 
is a good price for the cel. If another dealer 
had a similar cel, do you think they would sell 
it for $300 or more? If so, then that means that 
if you ever want a cel like it, you'll have to 
come up with the money someday. My advice would 
be to go for it. The cels that you have to 
sacrifice and struggle to get, down the road 
those will be among your favorites, you'll really 
feel like you earned the reward.

On the other hand, if the cel is overpriced at 
$300, if it's a case where you've missed out on 
other cels of this character so you really want 
this one even if it costs a bit more, then I 
would say pass on it. Those are the cels that 
have you kicking yourself later on. Every time 
you look at it you'll remember that you payed too 
much, and it will be harder to enjoy the cel. 

Either way, good luck with it,
Skorp