Hi @dew, welcome to the forum 
Buoyancy is determined by the weight of the volume of liquid an object is displacing, minus the weight of the object itself. An empty box could have a higher buoyancy than an equivalent block of buoyancy foam, but whether it does depends on the thickness and density of the box walls (and their shape), and the density of the foam being considered.
From a cost perspective, it’s generally more expensive to have an extra enclosure in the place of buoyancy foam, but the required wall thickness depends on the material properties, shape, and required depth capacity. It can also be possible to have extra room in an existing enclosure to increase the buoyancy (there’s an example here), which may reduce the cost increase by not needing extra components (e.g. end caps).
For a modular and configurable vehicle like our BlueROV2 it makes sense to have buoyancy foam and multiple ballast weights, because it’s much easier to tune the buoyancy and balance by cutting foam and moving / removing weights than it would be by changing the size or shape of an enclosure. For a custom-designed vehicle with known and fixed components it makes sense to consider more fixed buoyancy and weight distribution options, that can be part of the vehicle body design.