In which case a BSP geometry is expensive?

I’m thinking of an open world game where i will reuse some assets and i thought i could use the BSP for faster solutions in some areas.
Im guessing BSP geometry is expensive when it has n-gons, like a BSP cylinder that has n-gons for top and bottom sides.
So the first question = Does the engine turn n-gon sides in BSP to triangles, if not, is the expense came from it too much to give up on n-gon BSP 's and use them only with 3-4 sided faces?
Second = whether a bsp cube is more expensive than an asset cube imported from 3rd party program like maya or blender?

Hi SongOfDeath,

BSP geometry is essentially for blocking out your level and then replacing it with static meshes later on. There isn’t any n-gons in the BSP geometry because everything must be triangulated when it comes into the engine. What you are seeing is triangles that are not welded together. The best example to show you is the stair BSP (picture located below). If you export it to a 3D modeling program you can see exactly how it is made.

With that said, BSP are more expensive than a custom imported static mesh. It is because they are editable in engine and they also have the feature that allows you to edit the texture per-poly.

I hope that helps.

Thanks, TJ

well of course it helps, u explained everything, thanks