With small quantities of developer in drums, there is a high risk of loss of developer activity due to aerial oxidiazation. So err on the side of caution, and use too much to start.

I had the same concern that you do as to the cost of developer when I started printing ra-4 ( whell , actually EP/2, but that dates me, back when I was a cash poorer student). So I would do 'semi replenished'. My ilford tube needed 70ml to cover the print, but the print actually only needed about 35ml of chem. So I would start with 100mL, and funnel pour it from my tube back into a small holding bottle that in between runs would get dropped back in the tempering water bath. After the third print I would dump 35ml, and replenish with 35ml of fresh solution, and keep pouring off then going topping up for the balance of that printing session.

After a session no partly used developer was kept. The stock unused developer was topped off with 'private preserve' wine storage gas. The same gas was used to top off any partially used liquid concentrate bottles wahn partially mixing a kit ( think the kit I used was 5L, and I only would go thorough maybe 2L before the 6 weeks was up at times, and then the 'fresh' working solution would be dumped.