Capitalization does not cost shields. Yes, it has the same "turn-count" as 600-shield wonders like Apollo. IGNORE THAT "TURN COUNT". It will tell you that there is a certain number of turns before it's completed. THAT IS A *LIE*.
The brackets around the word (Capitalization) and the brackets around the turn count, like (50 turns) mean that Capitalization isn't really an improvement.
That's what we mean when we say you don't build it. Try buying Capitalization. You can't, because Capitalization isn't something you can buy. The game is LYING to you by listing Capitalization as an improvement. PRETEND IT'S NOT IN THAT MENU.
Instead, pretend Capitalization is a button on the city screen. Pretend that when you press this button, the "shield-box" DISAPPEARS. Now look at where your shields were. You will see they have turned to gold coins, meaning that instead of adding shields to a shield-box, they add gold coins to your money.
The reason why they put Capitalization in the improvement box is so that you can see that something is PREVENTING YOU FROM BUILDING. When you "hit the Capitalization button", you can't build anymore - all your shields are coins! - and they represent this by placing Capitalization in the box where you normally see what you're building.
The reason you might want to "hit the Capitalization button" is if the city's finished building. Let's say you don't want any more units, and you've built all the buildings you could possibly want. A city will still produce some number of shields per turn. Instead of building units and buildings you don't want, switch the city to Capitalization. As I said above, that city will stop building - all of its shields will become gold coins.
-Sev