I find an interesting transaction:
http://blockchain.info/tx/9106ac6859097079d39127aaac86208ac2a2b9bade92c3ae109192b7bc340872Paying 0.01BTC fee, it sent 228 x 0.000001BTC outputs to SatoshiDice. SD returned the fund (because they were lower than the minimum bet) with 228 transactions with 0.001BTC fee for each, with a total of 0.228BTC or about 3USD.
Comparing the loss of the attacker and SD, the ratio is 22.8x and seems quite effective. Actually there were 243 outputs in the attacking transaction (15 of them were not sent to SD) so the real ratio should be 24.3x. Since outputs do not contain public keys thus are small in size, I think 0.01BTC fee could actually pay for more than 243 outputs, making the attack even more effective.
So the question is, what could be the maximum harm done if the attacker paid 1BTC fee?