You can't do the math that way. You need to look at how much 1litre of working solution will process. OTOH I don't think saving a few pennies on developer makes much sense if you don't like the results.
Just to add more fuel to the fire.
Agfa MC is about $6 Canadian for 1litre locally.
Mix it at the normal 1:4 dilution and you've got $1.20 for 1litre of developer.
The yield is very high. So much so it's maybe 1 cent per 8x10.
Even if it was 5x that the paper costs alot more then the developer. So buy what works best for you.