its 10 silver for your charter. You must get your charter signed by 9 people. Make sure noone who signs it has signed one before or it'll bug and you'll be out 10 silver. Once your formed to design your tabard its 30 gold. Once you design it for a person to buy it its 1 gold.
It's 10g, and you can buy a tabard at any time. It'll be white until the Guild Master designs it. Not sure if you have to be in a guild to buy a tabard, but probably.