hi,
i reckon whatever you can afford is perfectly fine to give... my rule is always to try and find out roughly what the cost per head of the reception is and give about that... but depending on your budget this could be too much!

my husband and i asked for money in a poem for our wedding as well as giving option of ppl giving a traditional present... personally i think that if the couple lives at home and have never lived in a home of their own there is definitely something that they will need..

I found it rude when my sister in law and her partner asked for money when they got married when they had never lived together and didnt have anything that they needed for their own home... that made me a little angry, and i got them a gift instead because i think if u r going to ask for money it needs to be clear what it will be used for... on ours we actually made it clear that we were putting it towards a deposit for our own house so we could stop renting...

Anyway, good luck, only give what you can afford and if you are still uncomfortable with giving money the couple will not mind if you buy a gift!