Until Level 23, Fluctuating requires the least total experience except for at level 11 where it requires three more total experience points than Medium Slow. Then it ends up being the middle ground in terms of total experience needed. At level 46, Erratic ends up requiring less total experience than it. Finally, at level 62 it ends up being beaten by slow. It requires the most experience of all growth rates, requiring 1640000 experience points total.
All in all, not the best growth rate. Fast, Medium Fast, and Medium Slow are better for getting to level 50, and Erratic is better for getting to level 100 (the two important levels.)
Here's the formula (courtesy of bulbapedia)
Also check out http://bulbapedia.bulbagarden.net/wiki/Experience#Experience_at_each_level for a table of experience gain to see for yourself what the Fluctuating growth rate requires at each level.