Not necessarily, no. Are the best cards expensive? Yes, in most cases. However, it's still possible to make some very good decks for low amounts of money. It's all based on a player's knowledge of the game and their ability to put cards together into a deck that will work well together. You can put $5,000 into a deck and use cards like Black Lotus, but it won't do you any good if you don't maximize and efficiently use your deck's potential. My best friend made a Dimir deck for less than $10 and that could effectively cancel his opponent's spells and then shut them down with Cipher spells attached to Invisible Stalkers and other creatures. The strategy was fairly reliable and often did very well against all sorts of decks. Now, I believe that the most expensive decks can most certainly be the best decks, but that's not always going to be the case.