I usually just get a 20 point token when I complete one of his quests, but sometimes I get a 50 point token, as well. Does anyone know why this happens?

You get a 50 point token for your last quest.

you get a 50 point token if you complete the last quest. So that´s 10 for a regular account and number 15 for a gold one.

Ah, thanks! I can't believe I didn't notice that...