Am in a quandry about this one myself.
We are going to a friend's wedding in Ibiza in June and got the invitation the other day.
Usually for 'abroad weddings' it has said: your present is your prescence so have always given £20 - £30 regardless.
This one didn't say anything but a donation to the honeymoon. I wouldn't mind but they are having two weddings and her family is minted. Just seems a bit presumptuous.
As someone has already said - a lot of generous peeps on here
Generally think £30 - £50 is a good range unless its close family which should be more.
BB x