Depends upon the company, but for some it could be a lot. Personally I think it should be a percentage of the Company's revenue split between various Open Source Orgs.
For example, almost every company uses OpenSSH from the OpenBSD people, so companies should toss some funds to OpenBSD.
In 2023 I see Amazon, IBM and Apple gave 0 to OpenBSD, where Microsoft and Google gave a decent amount. I know IBM uses OpenSSH on AIX and Red Hat. Apple and Amazon I have to assume uses OpenSSH.
https://www.openbsdfoundation.org/contributors.html
So I think these same companies should split their donations between various gropus, at the minimum it should be FSF, OpenBSD and OpenSSL.
It's disappointing to see this undervaluation of OSS. The pledge to pay $2k/dev/yr is about meaningfully giving back to the OSS community that supplies us with invaluable tools -- tools which we build businesses on top of -- for free. It's about giving back for the many "perks" we partake in, even though many undervalue them or even feel entitled to them.
You're right that not every company can afford $2k/dev/yr, and that's okay. I think this pledge is for the ones that can.
There are other initiatives like https://fossfunders.com that don't have such a high bar.