Would you be interested in purchasing rare cards in the market?
Even if Valve is planning to avoid the prices of cards going through the roof, there's no guarantee that every card will stay at an affordable price. In time, every game forms a certain meta, and I assure you, it will make players crave for the best card. Especially when more cards are added into the pool, making it more difficult to pull the exact card you want, players' demand for a certain card will exceed supply in the market. That's when you'll see a crazy amount of money going back and forth among players. I even assume that there will be players dedicated only to the market seeking arbitrage opportunities. Personally, I wouldn't pay more than what I think it is worth, but I'll never know for sure until the game comes out.