No one said to LIE.
It's the opposite.
You quote high to inform them honestly what it would take, rather than deciding for yourself what is really their choice not yours. Your choice is to not take a job if you just don't want to or can't of course, but is not to tell anyone else what they want or what it's worth to them to get it.
So as long as you don't mind doing the job fundamentally, but it's just a matter of resources or time or efficiency etc, (you're not being asked to do something you morally object to, or totally unrelated to the kind of work you want to do, or requires something like living on an oil rig for a year etc...) then you have no excuse to do anything but make your best guess at the honest estimate whatever that is however outlandish you think it is, including whatever no-promises cya terms you think should apply, and just tell them that. "It will be $200k, 4 months minimum, and you might get nothing in the end."
And part of that is also giving your best honest advice about what they should actually do, and your reasoning. Some other approach that you think is better, or recommendation how to find some other supplier who could do the approach they wanted "that will require rust and high availability, so you want to look for shops that specialize in rust and high availability...", whatever.
The end result is that they are the ones who decide not to do it rather than you. And if they go for it, well, considering you weren't bullshitting them but giving your honest best info, recommendation, and reasoning, you take it and do the job. If that means spending 2 months just learning a new system or language and throwing away a bunch of experiments just to get started, or hiring someone else to do some part you can't do, so be it. You put all that into the quote, and they said do it, so enjoy your big safe gig for the next while.