Second, past 80 and you're on the wrong side of the profit zone curve, so even with donors giving a full 1.9x of each spot, it's still pretty expensive to level (and increasingly so every level). First, the rewards per level go way down after 80 (from 10 - 60 it's 1% increase in bonus each level 60-80 is 0.5%, after 80 is 0.1%).

The reason why level 80 specifically, rather than say 75, or 90 has to do with a couple of things. Others have gone over the benefits of the Arc generally. The higher figure is the amount you'd need to spend without rewardsīy using the Arc correctly you can lvl up 3 GBs to lvl 80 for less then what you potentially would need for just 1 of those GBs The lower figure for that is if when lvl'ing up you managed to get all the rewards back in investment. Lets say you want to take Zeus, CoA and CDM to lvl 80. This will massively reduce the cost to lvl up your GBs If you're able to put enough of your own FPs in to make positions secure then others can take those spots at 1.9. If the GB is low enough in Lvl I may even be able to get away with only putting 1 or 2 FPs in to reduce the loss if it doesn't lvl but have a massive gain once it does lvl. The more GBs I put FPs into the greater pool of FPs I have coming back to me that I can then re-invest into more GBs.

I only need enough of the GBs I invest into to lvl up that I'm gaining more then I put in. The Arc allows you to lock in a position and know that you got a increase of investment on those FPs once the GB lvls up. Without the Arc you'd be losing FPs on most investments unless no one else contributes to the same GB. With an Arc you can potentially put down FPs and lock in to gains hundreds of FPs from what you put on because of the Arc bonus. One method is limited by how much you can cut out, the other has unlimited potential although it also entails risk. It's like the difference between money growth potential if you cut out coffee from your diet or decide to start a business. Usually you'd be restricted to just your city collection which basically has a cap to it. The beauty of an Arc is it allows you to open up your options to multiple income streams with rapid exponential growth. If you're able to use it in a way that gets a return then it's worth the space it takes up. If it's not giving anything in return then it's 'incorrect'.

Forge of empires constructing the arc