For what it's worth, when I tried to submit my deck I got a Network Error and had to close the application. When I reopened, I had to build my deck again. I took a screenshot to see the differences if any, and the card I was hover-holding is now removed from my pool. I suppose it could be costly, because in this case I don't have a card I was trying to select and I didn't know I wouldn't have the card until the draft was over. Imagine building your deck around a card you don't actually have.
For what it's worth, when I tried to submit my deck I got a Network Error and had to close the application. When I reopened, I had to build my deck again. I took a screenshot to see the differences if any, and the card I was hover-holding is now removed from my pool. I suppose it could be costly, because in this case I don't have a card I was trying to select and I didn't know I wouldn't have the card until the draft was over. Imagine building your deck around a card you don't actually have.