I made a thread a few months ago about adding something like the Wonder Age for AoE2 where we discussed possible bonuses for Wonders in Conquest (there should still be a setting to turn them off completely).
I’d rather suggest letting them generate more Gold then making them cheaper.
Scaling the costs by player numbers might also be a good option in general.
500 Resources per Player (independent of which team they are on).
So 1000 like now in 1v1 but up to 4000 in 4v4 or a 8 player FFA.
It could provide some bonus to the entire team to make up for the increased cost. Giving one of your existing civ bonuses to your team mates might be a really cool mechanic.
If it trickles resources it should be like 5 Gold per second. Much more then those unique techs but they are also much easier to get and they can’t be destroyed.
Or maybe a very simple idea. You can trade with your wonder as if it were an allied market.
This way it would be viable in 1v1 but totally not broken at all in Team games. And it would just add a mechanic that already exists and is balanced to 1v1s.
Maybe the Wonder could add the bonus feature that you can choose to trade other resources (no Stone though).
Wood can be more valuable then Gold on Water maps. This way it stays relevant in team games too. Food would probably never be worth it though.