I remember reading that lots of CGI studios (which work for realistic movies) prefer having a renderer that’s predictable and reliable and to just let it do its job rather than spend time to optimize the render speed. That’s because the big cost is the artists’ working time. Studios have powerful computers and render farms to deal with the render times and those don’t need to be paid wages when they work.
That’s for realistic CGI. Anime style graphics should usually be much faster and easier to render than that.
But, someone who knows first hand could likely give a more detailed answer than I could.
Of course, if you double amount of images to render, you are increasing a lot, rendering budget.
But rendering budget may be very low compared to the rest of production costs.
Time of machines computation is less costly than human work time.
There is a big difference between animating with 30fps in mind, and just providing a 60 fps version by retiming files, and making an animation with 60fps in mind, in order to put more animation details in final result.
Each project is different so it’s impossible to give a one fit for all answer.
Many anime don’t animate all the frames at 25/30 fps anyway. But camera motion and parallax effects are generally animated each frames.
I don’t think the real problem is rendering time, at least the final frames, even if it obviously count.
But every step in the process will require more disk space and more processing power, and probably more people working time.
To give a precise answer for a particular project we should know the cost of each step and the exact process, to give a precise estimate that can take days since there is a lot of steps involved.
It’s really impossible to “categorize” any project: every one is different. You make individual decisions about how you want to set-up each shot, what rendering parameters you want to use, indeed which renderer you want to use. All of these have a very-direct effect on “time.”
However, these days, it’s also very easy to “throw silicon at it.”. You can buy time on render farms. It just costs money. Spend wisely.
In my workflow, I of necessity use the EEVEE render-engine, because Cycles produces too much heat on my gear. This has various effects on the render outputs that I get, and I accept them. You must do the same with your choices of how to set up the project and how to deliver it to the computer.
There are many ways to “get acceptable results, in reasonable time,” on “the hardware that you have.”