I used to give them one attempt at education as to why software development isn't off-the-shelf if it seemed reasonable that they might simply be new to that sort of project. In practice, though, it was almost always the case that they were simply being unrealistic and hoping to find someone new / desperate who would give them more than what they paid for.
How well do those projects go? I've had bids rejected for being too high, even when other developers failed to deliver due to underbidding. Some people never learn.
In a couple of cases, it went well – there are people who genuinely appreciate being educated about what they as a businessperson need to know about approaching software dev. Sadly, it's a definitely not a high success rate: if you don't get an impression that the conversation is going well, there's probably a good reason for that.
