To preface: I help make enterprise level web software for a living for a corporation who is new to software.
I have learned (at great expense) two important principles that apply here and may help you. So, your official answer is:
Apply the 'Make it Bluer' principle, and apply the Three Bears principle.
(Don't worry, I'll explain both.)
Make it Bluer
Simply put, this principle is about customers not knowing what they want, but "knowing it when they see it".
In other words, they cannot articulate what it is they want (because they don't have the vocabulary to tell you), but they can tell you what is wrong with what you put in front of them.
To apply this principle, make a very quick, cheap as you can, prototype that fits within the vague notions you have gotten from them about what it needs to do. This will be somewhat close to the "mama bear" solution that you will present in the three bears principle.
(Why is this called "make it bluer"? I once had a gig for a small company to make a corporate presence website. Their most important feedback was to make the text bluer. They didn't like how blue it was, and wanted it bluer. So I made it bluer. Then they decided that they didn't want it quite that blue, and could I please make it the blue I had it before.)
For a similar principle, look up Bikeshedding, which is related.
The Three Bears
(In case you are unfamiliar - this is the internet, and not all cultural references are the same - here's "Goldilocks and the Three Bears")
For the purposes of this principle, the Papa Bear solution is the "hard" one. It takes the most time, takes the most resources, costs the most, and is invariably the best, most ideal solution.
This is the "rocket to the moon" that they ask for. Present it with a due diligence estimate, and present it first.
The Mama Bear solution is very little more than a prototype, and just barely (or "bearly"? - DAD JOKES!) does what it needs to do. It can be a few patch ups on your cheap prototype for the "make it bluer" section.
This takes the least time, costs the least amount of money, and might just be enough for your customer. Present it second.
The Baby Bear is "just right". It rides a balance between Mama and Papa bears. You present it last, so that your pitch involves:
- Here's the Papa Bear solution, which fulfills your wildest dreams, and costs more money and time than you've got
- Here's the Mama Bear solution, which is cheap, quick, and kind of looks it
- Here's the Baby Bear solution, which doesn't look as cheap as the Mama Bear, but doesn't have the features of the Papa Bear. However, you can afford it, and it won't take forever to make.
I have used these principles and even the very terminology I present here in front of executives. Non-devs find these terms very approachable.