Like I said on the Steam forums, I'll pass and wait for the sale.
EDIT: Just doing some quick math here but if a game costs, say, 10,000 gems (starting price) and each card gives you about 20 gems, you would need roughly 500 cards just to start bidding. Since cards can be bought and sold on the market (for, you know, actual money), in order for the auction to make sense for the customer, cards should cost less than, say, $0,10 (at $0,10, a 10,000-gem game would cost $50). Bear in mind this is just a rough starting price we're talking about. The more gems a game costs, the lower cards should sell for for the auction to be a valuable proposition for the customer. In the end, however, the clear winner is Valve since games will undoubtedly sell for tens of thousands of gems that probably required several hundreds of dollars worth in cards.