Just so the non-crafters out there understand, high level bags require a rune of holding component that you buy from the trainer. This is in addition to the ore, leather or cloth required to craft it. That rune of holding can cost a LOT of money for higher level bags.
Rune for 10 slot bag - 5 silver
Rune for 12 slot bag - 15 silver
Rune for 15 slot bag - 50 silver
Rune for 18 slot bag - 2 gold !
Rune for 20 slot bag - 10 gold !
So, understand that when you put an order up on the marketplace for a 18 or 20 slot bag, you're asking someone in the guild to spend a TON of money to help make you a bag. It's essentially a waste of money to have much more than 12-15 slot bags until you've gotten to level 80 and acquired solid gear... otherwise that money and effort would be better spent helping you get better armor or weapons. For example, instead of an 18 slot bag, with 2 to 3 gold worth of materials, I could make someone two entire sets of rare (gold) armor for level 35, 50, or 65. Just be patient while we get more armor and weapon recipes added to the marketplace.