Is there a standard amount of XP given to a cleric who is successful at Turning Undead?
In 3.5, you gain experience for overcoming challenges, not for individual things you do. So a fighter doesn’t get XP for successfully attacking, a wizard doesn’t get XP for successfully casting a spell, and a cleric doesn’t get XP for successfully turning an undead creature.
Rather, they get XP when their attack, spell, or turn undead contributes to overcoming a challenge set before the party. If that challenge was combat, it’s pretty clear: if the attack helped kill an enemy, if a spell helped incapacitate an enemy, or the Turn Undead drove an enemy away, that action contributed to overcoming the challenge that was the encounter. It therefore helped them get XP. But the entire party gets XP for overcoming the challenge, regardless of how they did it.
And not every challenge is about combat. Perhaps Turn Undead outed the vampire hiding in the king’s court, or the spell deciphered an ancient prophecy, or the impressive show of martial prowess in the attack has the guards fleeing without a fight. These are also challenges worth XP.
Some DMs give bonus XP to party members who were particularly lucky or clever in overcoming challenges. Some DMs give bonus XP to party members who roleplay particularly well. Some DMs very carefully follow the CR rules in the book for calculating XP. Some DMs ignore XP altogether and then just say “you all level up!” when they have accomplished something particularly deserving.
There's no standard amount. I D&D 3.5e XP is dealt for overcoming challenges, not for doing actions.