I am a beginner at this, so bare with me please.

I am starting a collection of anime cels and so 
far I have three, all of Outlaw Star characters. 
My questions are...

1. Is it true that OS cels rarely have matching 
sketches?

2. If so, what are OS cels worth on average with 
a matching sketch?

3. Also, I am thinking about buying cels from 
Fist of the North Stara and Mobile Suit Gundam. 
Does anyone know what those tend to go for these 
days. 

Thanks! Below is a picture of an OS cel I just 
got today!