Thanks everyone for answer. Looks like I need to install Lightheaded. It's not a 100% answer but fit for first time.
May be any of addon maker would like to add this functionality to their product? I think it would be some quest-database addon (like Lightheaded) 'cause I think there is no way to know quest XP reward until you complete quest. May be even Lightheaded can add such functionality.
To egingell: XP scaling up and down according to you level. Look at wowwiki.com for more details :)