The correct verbs to complete the sentence are "is" and "thinks."
So the complete sentence would read: "Two billion dollars is a high price for a satellite that half of the scientists thinks will fail."
This is because "two billion dollars" is treated as a singular amount. "Half of the scientists" is referring to a collective group, and in American English, we often use "thinks" in this context to refer back to the collective group, even though some may also argue for "think."
Therefore, the correct option is: is; thinks.