How about looking at it this way.
Accept your assumption that 1.5 million is too much, representing the extreme upper limit.
But coming from the other side, we can be certain the amount Obama has spent defending against the birthers is greater than zero.
What then can we speculate would be the extreme lower limit?
Say $100,000. Many would scoff and say it’s way too low. Anyway, a rough estimate can be gauged through the means described in these discussions.
Now take the lower limit and the upper limit and establish a reasonable middle value.
The answer here, and we’re being extremely conservative, would be an amount approaching one million dollars and steadily increasing.
Okay. $1.5m is too much. $100K is too low. I still can’t get to $1m. I just don’t see any evidence. I might be willing to go $500K.