I was told that the tradition with giving money at a wedding was to give 2 envelopes - 1 with a card (which goes on the table) and the other with cash but no card (which goes in the wishing well).

$150 pp seems incredibly ludicrous. If they are expecting that from each person at their wedding I would think they have a snow flakes chance in hell of it actually happening. Most people would not give that much. It is an interesting concept though - a wedding as a fundraising event

I think $50 for each adult is a reasonable amount. Unless you are particularly close to them and want to give more.