Well, I'm glad you cited "math issues" Elizabetty but that's not the only math involved. While it's true that Romney has deep personal pockets & therefore can stay in as long as he likes, there's 2 diminishing aspects to a losing campaign:
(1) The income ratio changes. If Romney was spending, say a $1 of his personal fortune for every $4 coming in elsewhere...that could change to him having to spend $3 for every $2 coming in elsewhere...especially with big media drain states like CA & NY coming up. At some point, Romney has to ask how much of his fortune is he willing to stake on this attempt to personally purchase the presidency.
(2) The business analysis side of Romney the investor has to kick in to a greater degree. Investors know there's a thing called a "return on investment." If you are pouring mucho more than all your competitors put together in an investment, & each wave you're not getting the best "return on investment," the "cost per vote" calculation eventually becomes obvious...it becomes "too expensive" to garner the number of voters you need to win the whole thing.
If your campaign is doing well, then you don't have problem #1. If your campaign isn't doing well, then problem #1 kicks in and exacerbates problem #2. Voter cost "inflation" makes winning the whole thing a HUGE gamble of a good chunk of your own personal fortune.
If the final choice is Huck, McCain, Rudy, or Romney, then I will have a choice to make---but I can just about guarantee you it won't be Ron Paul, Duncan Hunter, or Fred Thompson.