Yes, you could theoretically calculate whether it's worth it ...
I am a very rational person and I have been buying these STABILO BOSS ORIGINAL refills for many years now. In my opinion, only practice is relevant to this question:

The question is not how much space there would be in it, but how much liquid colour goes through the pen fibre!!!
If a STABILO highlighter pen is a little older, the pen fibre is no longer as absorbent. It can then happen that the highlighter pen spills if you try to pour in too much colour at once.

I have just tried this out again;
I refilled my 4 yellow STABILO pens (in 3 sizes) with an ORIGINAL refill. With one pen I pulled it off too quickly and then the refill didn't close properly. The next one made a yellow mess!
-> I'll have to replace one of the pens AND get some kitchen paper ready next time.

CONCLUSION:
+ If you need STABILO highlighters so often that they dry out within a year, I recommend these STABILO BOSS ORIGINAL refills.
- Otherwise, it is better to replace the highlighters from time to time (especially if the pen is dirty).

I've found different information on different sites. Some say 3ml and 2 others 9ml?
I think it would be useful to write directly to Stabilo.com and ask for the actual quantities of Stabilo and the refill!
Based on my experience: the refill lasts well and "quite a long time" and I'm happy with it. I can't quantify it.
I am happy to throw away only the refill and NOT the whole Stabilo. Thank you, Patrick. Patrick.