Similar XP should yield similar power. That sounds simple, but in the OGL version of D&D, that is not easily achieved as the classes are so imbalanced. Fortunately, we can price the classes so that the bang for the XP buck is less imbalanced.
For the druid, we can balance the XP buck by splitting the OGL version of the class into three tracks, each of which requires XP to advance. This solution works nicely for a pure druid, but once you mix in prestige classes, works less well. The tracks are Druid Spellcaster, Druid Beastmaster, and Druid Shapeshifter. In truth, we really have three classes smashed into one. They should be split apart and that will really solve all the leveling issues, but I'm here pricing the existing class, not designing a new one.
At first level, pick the type of druid that you want 1 level in. That's what you get. Whenever you go up a level, that track automatically levels with you. To get the other tracks, you need to pay the XP cost for that track as if it were a level (but you don't get any additional hit points, skill points, or other leveling adjustments).
As you should be able to calculate, keeping up all three tracks on druid would be damned expensive, which is the point.
The problem comes in with Prestige Class that give you additional spell levels. There are so many mechanics with Prestige classes that I can't track them at all. As a rule, any prestige class can only help one track per prestige class level. In most cases, this should be clear. A shapeshifting prestige class should progress the shapeshifting track, while a casting prestige class should help the casting track.
Roughly speaking, this should increase the cost of being a druid by three times. Considering that a well built druid can do the work of three characters, this is good price match. The druid's overall level, if he wants to stay at full power, will always remain one or two levels behind that of the party. At the upper ends, the druid will still hit runaway power.
As an additional rule, beastmasters may replace their beasts for free at any level increase, but between levels, replacing their creatures requires an XP expenditure of some sort. I haven't worked out the math. The cost should be about 1/4 of the current level's experience cost. So if a 2nd level druid were to lose her creature, it would take 250 xp expenditure (as 2nd level requires 1000 xp) to replace her creature in an ad-hoc manner. The druid may go into debt for this. That should prevent any abuses associated with careless creature handling.
As for shapeshifters, the druid should gain one form per level. Each additional form should cost 1/4 of the currently level.
I'm sure that there's a loophole in there somewhere.
I've you're saying to yourself, "This makes druids suck," then I know that I've solved the problem.
Overall, I fear that I haven't taxed the druid enough. All that I've really done is to delay the super-druid for a few levels.