You could use a geometric series, for example say that to get from level to level will be times more than it takes to get from level to .

In equation form it would be . You can pick a suitable value for a (the starting value) and for r.

For example if a = 100 and r=1.2 you'd need (total) experience points to reach level 3 and (total) experience points to reach level 10.

To get from level 1 to level 2 would require exp but to go from level 8 to 9 would need exp points. Increase to require more experience points to level up.