Quick mental math: 2.5k transactions or so in a block, 3 cents each for transaction fee means that it costs $75 or so to fill up a block with economically irrelevant transactions (i.e. moving money from one of your pockets to the other). This means you can degrade the performance of the Bitcoin network for about $10k per day.

You can probably optimize that down to $3~4k or so by being smarter about the transactions you're DDOSing with. This is left as an exercise for the reader.