I've seen card cages before, I think they're pretty (although overpriced! Sheesh, I about died when I saw the price on
this one). For a bag, though...hmmm.
Wedding money bags tend to be just rectangular drawstring bags made out of satin or some pretty fabric, but this one probably isn't going to be carried around, is it? More likely set on the gift table so that people can put their cards into it?
What about taking a standard paper gift bag from the drug store, cutting it apart carefully to use for a pattern and then making a fabric version out of a pretty sheer? Then using the cardboard square from the bottom of the fabric one to stiffen the bottom and a framework of silver (or aluminum) wire to stiffen the sides? I'm not describing this very well, but you could form the wire into a rectangle the same size as the open top of the bag, and then make four 'legs' that slipped down along the four corners to hold the bag upright and open. Then just put on a handle of ribbon or decorative cord and you've got a pretty standing bag that people can see the cards inside.
I think I've been more confusing than helpful, but if you want me to strive for a proper explanation of what I'm thinking, PM and I'll try?