Here's how we know it's more than $100 million. Last month, Romney's cash on hand was $7.7 million. That means he spent $400,000 more than he raised during February. He raised $11.5 million, so his total spend is $12 million. Through the end of January, Romney had spent $55 million, so his total campaign spending through February was $67 million. Meanwhile, his Super PAC has spent a bit more than $33 million—bringing us to more than $100 million total.
As you can guess, $100 million dwarfs Romney's Republican rivals. We don't know exactly how much Newt Gingrich or Rick Santorum spent through their official campaigns in February, but if you add up their January reports along with their Super PAC spending (which is up to date), all other Gingrich spending totals roughly $34 million and all other Santorum spending totals a bit more than $10 million. Based on those numbers, Gingrich's total spending is at most $40 million and Santorum's is at most $20 million. Therefore, Romney's is spending somewhere between three and ten times as much each of his rivals.
Romney has almost certainly outspent President Obama as well, though by a smaller margin. Through the end of January, Obama's campaign had spent $63 million and his Super PAC spending still hasn't crossed the $1 million threshold. Unless the Obama campaign spent more than $36 million last month, Romney is in the spending lead.
Obviously, the big difference between Mitt Romney's spending and President Obama's spending is that Mitt Romney is spending almost all of his cash to destroy fellow Republicans in a primary campaign while President Obama is building an infrastructure to support a general election campaign.
Romney's cash is primarily funding an air war in which he's carpet bombing his rivals because he can't win on his own merits—and even though he's spending many times as much as his opponents, he still can't completely seal the deal. In addition to building a general election organization, President Obama's spending is mostly going to strengthen and expand his base of support.
Even though President Obama has much more cash on hand than Romney—$75 million to $7.3 million—Romney won't ultimately be at a spending disadvantage if for no other reason than that his Super PAC can take unlimited donations. But there's one thing that money cannot buy, and that's time. The longer it takes Mitt Romney to secure the nomination (and as Markos pointed out a couple of days ago, it could take a long time), the deeper his unpopularity problem will get—and the less time he'll have to buy his way out of the mess he created.