Thanks girls! I thought it was a little unusual - they are expecting ~ $150 per head (so $600 from our family....even though we have 2 little people) which gave me a bit of a fright.
I am a little sentimental and would love to buy them something special for their house - rather than a cash deposit into their bank account (which was the direction in the invitation). I am really uncomfortable with the whole money thing especially after the inital conversation.