Lets imagine your mine with half hashpower. Lets imagine that a block can contain 6000 transactions. Attacker has 1/2 hashpower. Offered load is 4000 tx/block.
Attacker crafts 2000/tx block at 1coin/tx fee level. Making the rest match him (plus episilon, which we'll disregard).
His average cost for spam is 1000 coin/block (2000 * 1-rate).
His average income is 2000 coin/block (4000 * rate). (He doesn't get income from his spam, he saves its cost however; see prior line)
His net income is 1000 coins/block, on average.
Well, what if the attacker's block is orphaned, and other pools pick up the now-unconfirmed transactions with 1 BTC fees? Would the attacker be able to prevent that?