Get the FBI to pay for Apple's time?
If Apple have declared that there is no way to crack the encryption, as backed-up by their spec testing, and if they honestly believe that to be true, then I see no problem with Apple attempting to crack it. By their own declaration, they will not succeed.
However, given that they have declared this, and have the testing to 'prove' the uncrackable nature of the algorithm, why should they be forced to pay for the cracking process?
If the FBI insist that Apple have a go at it, let the FBI pay for the engineer time out of their own budget. That would likely put a natural cap on it somewhere, because someone would be bleating about the expense fairly rapidly, I would think.