1. Obama has already spent hugely — to the tune of $400 million! — to define both himself and Romney, setting the narrative for the race to come, no matter how much Republicans can spend between now and Nov. 6.
2. Because of the incessant news coverage of the race, candidate — and even outside group — spending carries significantly less import. “There is so much free media coverage for both candidates, and even more for an incumbent president, that no amount of paid media can come close to matching it,” said Jonathan Prince, a senior Democratic strategist. “What’s critical is to drive message, to own your image and define your opponent’s.”
3. The law of diminishing returns applies to politics. That is, if Romney spends $850 million total on the election to Obama’s $750 million, will it make that big a difference?
All great points, and I think it's particularly important to also note that Romney needs to introduce himself to voters and that costs money; whereas, Obama obviously needs no introduction and can focus on other things.