Slitherine

Prestige is rarely a limiting factor, whereas core slots are, so it's not really worth looking into the cost-performance ratio for prestige as the possible savings have no meaning. Of course the situation is different in single scenarios, but I doubt anyone uses elite reinforcements there.

FWIW, a 6 strength 5 star unit will outperform a 0 star 9 strength unit (5.4 hits vs 4.5 hits on average), assuming they have enough of an initiative bonus to fire first. Things get a lot more complicated if the enemy gets to return fire.

I have reached Sealion Bridgehead on Colonel with about 18K prestige. At General, it is proving a challenge to have enough prestige to buy the rail gun at Sevastopol, select the non historical path after Stalingrad. Moscow, Stalingrad, Along the Volga and Kuybshev burn through a lot of prestige.