We didn't offer anything when our greatnieces were Christened - there was a collection plate for the Church, but I wouldn't have even thought about giving the minister a tip.
Don't know how they do it now but when I was an R.C. Alter boy (don't ask how long ago) weddings, christenings etc were a set fee usually paid before hand but if altar boys, choristers were required they were usually tipped separately. At that time it used to be 2/6p a boy, doesn't sound a lot now but back then it was decent money
Thanks for your answers
Paddy - all the time both my boys were altar boys they never got a penny but that's cause they only have them at mass now and not at other sacraments
I don't know about the Church of Rome, but in the C of E there is no charge for baptism although there will be a collection plate towards the cost of upkeep of the church. The priest will not get, nor will he expect, any kind of emolument, though I know a couple of vicars who would not say no if a bottle of the hard stuff was discreetly pointed in their direction!