If you donate enough money to a Temple you will receive a Blessing. In MM8 this is always Hour of Power. The number of donations required to get the Blessing depends on the day of the week --- ie, 1 on Monday, 2 on Tuesday, etc. The power of the Blessing goes up as the donation does, but don't try to make sense out of the numbers; it doesn't work the way that you'd expect the Light Magic Spell to work. It will have to be sufficient to say that the value of the Blessing goes up proportionately to what you donated.
You'll also want to know that the component Spells cast by a Temple's Hour of Power will have priority over those cast by you. If you can cast more effective Spells than the Temple can on a given day, save your money.