There is a really easy way to tell how much a favor an item is worth. The favor is 1/10 of the item value. So say you want to donate one piece of meat. Meat is worth 10 currants. You will get 1 favor. Or for larger amounts of items, for 10 of an item, you get one item's currants worth in favor. So, if I donate 10 pieces of meat, I get 10 favor. For more stuff about it, try to find Zu's post on all the donation stuff,
Hope this helped :D
Click on the item in your inventory.
Click on "info" in the upper right corner of the box.
Look at the line "worth about xxx".
Drop off the last number.
That will tell you approximately how much favor you will get from one of those items.