One key intangible benefit that may be learnt from the agile approach to fail early / fail often is that of learning. By this I mean the project team learns how to operate but also the business learns what the change could mean and how their requirements could be realised.
It's possible to articulate this as a tangible benefit in terms of cost avoidance but this is difficult to articulate. The cost avoided from re-implementing around an error or the value of implementing a right solution is hard to calculate.
The real value of learning should not be underestimated however.