Unless a significant portion of the internet does this, and we’re talking hundreds of millions of pages, the only cost here is to you.

Fun twist: no! There's a very neat trick you can do when you serve the crawlers poison: you can hide an identifier in the URLs you serve them, and you can then identify that id when they come back riding on the back of remote controlled chromes. By serving them garbage, you can overload their queue with poisoned ones, which helps you block crawlers that you wouldn't otherwise be able to block.

Generating and serving garbage is incredibly cheap (cheaper than serving a file from a filesystem on SSD, in most cases), and once you have requests landing on poisoned URLs, you can firewall them off for a day or so, and reduce your costs even more.

We may not be able to poison the models, but we can poison their crawling queues. I have a year's worth of data to support that. They still haven't caught on.