This is unrealistic. There are many startups that are worthless (product is terribly architected, teams don't get along, they cannot track their customers revenue, etc.). A company cannot be expected to offer money for something without some minimum level of due diligence.
again, if you absolutely are not selling, then say that and move on.
Until you get your exit cash-in-bank-in-your-account, those Big Names are your competitors or potential competitors. The breakup fee is to compensate you for sharing more than just product name, price, and terms with them. Simply enthusing a little bit beyond "business for me is going great!" to a potential competitor who is in a related space can be sufficient market information for them to convince them to enter your space.
There is a lot of emphasis on people sharing tech/trade secrets, but keep in mind that simply confirming that users get excited by what you are doing in your space is information that these Corp. Dev. folks would dearly like to have. Never be afraid to say "no" to a one-sided deal.
Look at it this way: most Big Name Corp. Dev. won't even get out of bed for any deal less than an accretive $5-10M+ annual cash flow, and something that pencils out to much, much bigger than that 3-5+ years forward. If they are absolutely serious about acquiring, then they will start talking numbers soon after the NDA counter-signatures are exchanged, because no one wants to lose nearly guaranteed accretive income/revenue/users/etc.
If they are shopping you and your direct competitors to see if someone is naive enough to talk without compensation or willing to sell out for a relatively low number, then you see this behavior we're talking about in this thread. Whether or not you want to participate depends upon your personal exit parameters. This thread points out that everyone who might be in touch with these Corp. Dev. folks should already have some numbers in mind already, and a way to qualify the serious ones.
There are many startups that are worthless...again, if you absolutely are not selling, then say that and move on.
Except the problem is, most founders have a "I wouldn't sell for $X but would for $Y" and the harsh terms for discussion immediately let the founder find out if CorpDev is 1. Serious not just doing business intel and 2. Willing to pay the right amount.
If the firm was totally worthless then generally no one would be calling anyway.
I'd say that Corp Dev, shouldn't be looking at your company in the first place unless they have some confidence in you, and the break up fee you ask for sets the level of confidence you require for any kind of talks.
It also lets them know that they can't just bullshit you for several months while they gather confidential information about your product for their own development teams to copy later.
Clauses could be added for anything that would obviously break due diligence that the sellers may know in advance. I am thinking of IP, Patent, Contract problems etc.
"product is terribly architected, teams don't get along, they cannot track their customers revenue" - none of these things make a startup worthless as they can be fixed.
again, if you absolutely are not selling, then say that and move on.