1-3: 10% exp increase per level
4-6: 15% exp increase per level
7-9: 20% exp increase per level
10: 25% exp increase per level

MP10 gives you 160% more exp.

Edit: It should be mentioned that these bonuses are additive, not multiplicative. The actual gains you will see percentage wise are far less than what you would think. For example, the difference in EXP between MP 0 and MP 10 is 160%. With 5 stacks of NV and a Radiant Star Ruby, this would be a difference of 106% bonus to 266% bonus. (1+2.66)/(1+1.06)=1.78 or 78% more exp for MP 10. Not even double!

That, good sir, is ridiculous. I would rather farm at 1-3 because with high enough dps you are going to kill enough to make up for the shortfall in potential exp gain in the higher MP lvls.