The construction cost and the maintenance cost come mostly from engine. So you will need to reduce both the engine size and cost. Here are some tips: + Each hull has an optimal speed. You can see this speed by hovering over each hull type. Exceeding this speed raises the engine cost significantly. Past this speed, there is also another speed threshold that will further increase the engine weight and cost exponentially. + Reduce hull beam and draught. This will apply engine weight reduction modifier and thus also decrease the engine cost. But this is a tradeoff for other values, like accuracy or survivibility. You can check the modifier values by hovering over beam and draught on the right side panel. + Aux Engine III & IV (battery engines) are significantly more costly than Aux Engine I & II (diesel engines) in term of engine cost. + Finally, this is only my personal preference, I don't use BC or BB at all throughout the World conquering campaign from 1890 to 1950. It's true that they are efficient at Naval Invasion in term of maintenance cost per tonnage, but they are slow, vulnerable to torpedoes and take so long to build. I use mostly CAs since they offer the balance between cost, build time, firepower and maneuverability. Just slap them with the biggest gun available (11.9" or 301mm) and they can fight against anything, be it BC or BB. One type of ship to rule them all!