when we initially booked our church the priest gave us a leaflet of everything that needs to be done legally/religiously speaking and on it, it said donation of €80 to be given at time of booking church! now we havent given him anything yet but fully intend on giving him €200 the night of the rehersal!
I think it very much depends on your priest i know that my h2b parish priest was being a right pain in the rear as he said that we had to donate to the church every sunday or he wouldnt sign off on h2b now we dont even go to mass in that parish we are like 2 metres over the parish border! and we always go to my parish for mass..... he has since calmed down a little as i think another couple reported him to the bishop!!
don;t forget as well you have to give them sarcrestian (spelling woeful) you have to give them money and if you have an altar server its nice to acknowledge the fact that they were there too!!
hope this helps!!
p.s. with regards to the other priest coming! you only have to give money to the priest that is named on your marriage cert as having officiated at the wedding if you have more than one priest you dont have to! we are having 3 and tbh i think they are coming for the meal as well so that will do them!!