I've seen this graphic and as much as I would love to do this with every single one of my Reddit accounts if it truly cost them money, it is not plausible that it would cost them all that much money and I have yet to see any evidence of it costing them that much money. Only conjecure about how someone not connected to Reddit thinks it will cost them money. They are a software company, and as a software enginer (Reddit have a lot of those), it would be child's play for me to automate these requests and since the function exists as a result of GDPR regulations, they almost definitely had a system in place to do it.
But I would love to be corrected with evidence, do that and I will submit the requests myself.
In theory you could wait for evidence that this costs them time and effort, or you could do it proactively. In the situation where it doesn't cost that much, it doesn't cost you much because it's literally two buttons. In the situation where it costs them time and effort, well that's the goal so awesome.
As someone who works as a software developer, I know that not all "automation" is created equal, or set up correctly, or "fully" automated. Considering they give themselves 7 days to do it, they are likely responding in a ticket queue of some sort, so even if it's not super time consuming, there likely is manual intervention somewhere.
It's two buttons on our side. It's worth it to me. Also, this way I can copy down my history somewhere off Reddit, so that I can search it myself without going back there.
I don't think it costs them money out right but i think they're backend is hosted at aws. All these requests need processing power. That's where the increase in cost to them comes from.