you can go as thin as 0.7mm thick and without shapeways giving us some kind of discount for business cards they just are not to feasible. I would be willing to pay $1 each if I could make anything within a 85x50x3mm box. I would even be willing to pay double up front as long as the end price was only $1 each. Any more then that is just not reasonable for a business card.
A standard flat card one works great also. 3d would just be cooler and give an example of what you can do and in my opinion be more likely to bring more customers. more customers means more profit for shapeways so subsidizing the cards will pay off in the long run.
they could even add the restriction that the shapeways url must be included(I.E. shapeways.com or shapeways.com/shops/propmodule or any other url as long as the words shapeways.com is part of it.)
My guess if you take my suggestion. 100 cards will bring in $200 up front. there cost to you is probably about $80. eventually customer will use the remaining $100 for prints but you will probably not lose much as most of that money will go to markup.
Now if 1 in 5 cards handed out results in a sale of $25 that means for the small lose on the cards you will bring in $500 future sales.
I do not know your actual cost ratio but you do and you can do the math yourself and offer us what you think is reasonable. Don't be afraid to possibly lose some money up front you will likely get back 10 fold. And people will likely make there cards pretty thin for the most part since they need to fit in peoples wallets.