I want to make a quest that starts out by you donating so much gold to an alter. As I want this to be a huge amount of gold (500k+) I think people will do it over a long time instead of all at once. I was also thinking about different quests for lower amounts as well, say one at 100k, then 250k then 500k ect...

How can I keep track of how much gold a player has donated?

I would add an XmlValue attachment to the player with your quest name when they start the quest, and then you could just increment the Value property on it when they donate gold, and delete it when they complete the quest.