Make them with chocolate. [IMPORTANT: When I talk about chocolate, I am talking about confectionary coating/candy melts; NOT REAL chocolate with cocoa butter that must be tempered!] If you purchase 50 pounds of chocolate (candy melts) you would have 2 ounces of chocolate buttons for each of the 400 people.
Merckens candy melts (in their Rainbow line of products) come in different colors. The wholesale size box is 50 pounds.
Go to this link and scroll down until you get to Compound Coatings. The "chocolate" you need to buy MUST be a COMPOUND COATING and have a Merckens product number starting with CF-________.
Here's an example of a button candy mold:
Chocolate buttons would actually be very quick to make and will store easily in a cool room that doesn't get above 75 degrees.
As far as a price quote: Custom chocolate work LOOKS VERY, VERY EXPENSIVE! Do not tell your client that it is very, very easy and quick. I would research the cost of your ingredients and supplies, factor in the time, and charge an arm and a leg for the custom chocolate buttons. Even more if you are providing similar wedding colors with the chocolate buttons.
Are you responsible for locating, purchasing, and packing the 400 chocolate button favors into darling little containers? Every additional step makes the price go up and up and up!
Do NOT hesitate to quote a mind-boggling price quote!!! If they can afford 400 guests they can spend thousands on party favors. Even $5 a favor (fairly cheap for packaged, custom chocolate buttons in the wedding colors) would be $2,000.