If your church has no set fee, your 'donation' amount is usually at your discretion. If there is a fee, again it's at your discretion whether you want to contribute anything extra.
Agree. We're getting married the Catholic way in a cathedral too and the fee is $1250. Apart from that we plan to give a small amount to our priest as a thank you gesture, though at this moment we're not sure how much yet. I've heard some of my friends say it doesn't even have to be in the form of money, it can be a gift like a bottle of Scotch whisky. It really depends on how well you know the priest