Huge contract? Not really. Especially when many larger clients whom you'd love to have for a reference will demand it. Take care of it up-front and don't wait until you get into contract negotiation. If the client knows you don't have a software escrow service already in place they may ask for the moon. Head that off at the pass and have it ready to go from day one.
In fact having it ready to go might help you clinch the deal. If you're part of an evaluation and you're ready to proceed and your competitor has to go and get this setup - you're more likely to get that business. Likewise if you're the competitor who's been caught with their pants down then you're more likely to lose that business. Every salesman I know is always looking for something to give them an edge for closing a deal!
But I don’t have to please every client and don’t have to jump through hoops for small dollars. It would depend on the product/market if this was the normal expectation then I’d find that out rather quickly and may decide differently. But I’ve never been asked for anything like this and would gladly just say no if it was a small dollar value at stake.
This is a very standard thing to (a) do, and (b) be demanded in a contract.
It's not zero effort, but it's not that hard. There's plenty of firms provide the service.
Every time my former company shipped a release, we'd bundle up the source code, libraries, build scripts, etc, burn a DVD and send it off to Iron Mountain. Every time you do a sale that wants escrow, you add their details to the list and send that off to Iron Mountain too.
These days, I'd probably ship a Docker or VM image from the CI system: you need to give the customers something they can use to create their own builds.
We had some customers who actually tested the process, and when the company was sold and the product canceled, one customer took the option of getting the source code, and continued to make their own builds for a bunch of years afterwards.
In fact having it ready to go might help you clinch the deal. If you're part of an evaluation and you're ready to proceed and your competitor has to go and get this setup - you're more likely to get that business. Likewise if you're the competitor who's been caught with their pants down then you're more likely to lose that business. Every salesman I know is always looking for something to give them an edge for closing a deal!