Dont change the cost!!! Instead change what is unlocked!!!
My idea
Building just the BBQ should unlock just zhuge nu access. Vice versus building just the Imperial Academy should unlock granaries acess. And of course Building both gives you granaries zhuge nu and finally the -35% villagers production buff.
Likewise: Yuan dynasty
Clockwise Tower unlocks pagados
Imperial Palace unlocks fire lancers
Both unlocks 15% movement speed
Finally Ming dynasty
Either singular Imperial landmark unlocks grenadiers
Both unlocks 10% hp military buff.
The change to zhuge nu being unlocked with bbq would marry well with early feudal aggression. Meanwhile a Imperial Academy fast castle agenda would not suffer as much during farm transition if they have granaries as oppose to mills and therefore can keep up the tech momentum. Similar the situation with pagodas which sometimes go unuses until Imperial age has already been reached? (Some might argue that’s a reason to move ot back to ming with its former glory??) But if pagodas were unlocked with clock tower, pagodas would get immediate use. On the flip side a fast castle into imperial spies and fire lancers raids would be sickly…(maybe broken???), you wouldn’t have the horsemen speed out the gate but 1.62 knight like speeds is still noticeable if you can catch some vils slipping!!
Finally the grenadiers unlock would not be overpowered given how nerfed they’ve become and the fact that they would NOT inherently have the 10% hp buff. Heck nerf grenadiers hp down to handcannoneers hp of 130hp.
Simple changes that would immediately impact gameplay but would not change the necessities of Building both landmarks.