DEFAULT REWARDS 55 + 55 bonus
I wouldn't be worried about doubling your profit, you don't need to double it to get decent profit. 100% bonus would require a Lvl180 Arc which costs somewhere between 1,632,776 - 2,611,220 Forge Points (with 1.9 VS the entire cost). Barely anyone reaches that. Not because of plausibility, but because it's a lot of work and people tend to get bored doing the same thing over and over or quit playing the game altogether before reaching that point
What ARC Level ..starts getting good returns
Lvl 0. Getting returns has very little to do with the Arc level itself and more to do with the manner in which you use the building and how often. You can get decent returns on investment without a Arc. You can get far better returns with a Arc and usually in very specific methods
If you want to participate in 1.9 then ideally you want a Lvl 80 Arc. Where you go from there depends on what your priorities are and how long you intend to keep playing. If you're not going to stay around for long you might want to develop the rest of your city instead of focusing solely on the Arc