look on the wiki page for each class, it will tell you what the total xp penalty is for each race/class combination
edit: might as well post it here
- Dark Elf Shadow Knight: -40%
- Erudite Shadow Knight: -40%
- Human Shadow Knight: -40%
- Iksar Shadow Knight: -68%
- Ogre Shadow Knight: -61%
- Troll Shadow Knight: -68%