[knappe]

Keep in mind that the lifetime of this mission is actually pushing other missions out of scope. The budget needed to continue support of Opportunity could be dollars spent elsewhere. So, overspending doesn't make sense. They're roughy, $128 million (combined for both rovers) over their operating budget [0].

[0] http://en.wikipedia.org/wiki/Mars_Exploration_Rover

[cbr]

It was $820M to send the rovers to Mars, and $128M to extend that by 10+ years; the main cost was in getting them there intact. Yes this money displaces spending on other missions, and there are diminishing returns for applying the same tools to the same small part of Mars, but mostly these years have just been bonus.