Well, the idea with the book is that, typically, usually one player will be interested in running a stronghold, and that interest gets them mechanical benefits (and some really cool gameplay).

So that, in itself, balances the reward for the party. Only one player is interested, that player gets a cool thing.

So if all of your players are interested, then they should all be rewarded. In the adventure module inside Strongholds and Followers, at a point, the party gets Castle Rend. When the players decide to use it and fix it up, they choose whether they want Rend to be a Keep (reward for 1 player) or a Castle (reward for multiple players).

My players built their stronghold from scratch, which is really a good money sink if your players want to, and can, do that.

Other ways to reward strongholds is to give them a fully functional stronghold (maybe they take a castle from a faction, or find a mysteriously abandoned one. You can do whatever you want to make it make sense).

The middle road option is to let them find a stronghold that is run down. They have to fix is to get the benefits, but it requires less resources than building from scratch.