I really don't know much about personally drawn
sketches and know even less about Evangelion but I
know the cels are pretty expensive.  I guess it
depends on the quality of the sketch.  Does it
have colors like Pharna said, does it have
Yoshiyuki Sadamoto signiture, is your name written
on it?  All these things and more can affect its
value and the desirability of it.  It's probably
hard for people to give you a ballpark figure
since they don't even know what it even looks like.