The brother-in-law asked three contractors to give him a quote what would construction costs for the pool be. The first contractor came for a visit, measured the extend of the pool, and provided a quote a few days later. The second contractor did the same. But the third contractor, an experienced guy, didn't just measure the extend of the pool but additionally asked Lister's brother-in-law
"Do you know what's under the grass?"
If there's soil under the grass, a simple digging would suffice. But if there's a bedrock, dynamite would be required which would substantially increase pool construction costs. Realizing that the first two contractors provided quotes without merit, Lister's brother-in-law selected the third contractor for the job even though it was the most expensive of the three and it required to first do a test drill to check what's under the grass before providing a quote on costs of pool construction.
I hear time and again complaints from software vendors about selling agile approaches to government or large enterprise organizations who insists on fixed costs, time, and scope contracts. It might very well be that an agile approach could actually be a boon to your selling efforts, not a snag, and it is time to revisit the competence of your sales people, not question merits of agile approaches.
Let me conclude with another brilliant quote by Tim Lister
"Fixed costs contract with unknowns? Good luck!"