You have it backwards. Fusion will have much lower volumetric power density than fission, for fundamental reasons (one can circulate coolant through the core of a fission reactor, but not a fusion reactor). Fusion reactors will likely be much larger, and therefore much more expensive, than fission reactors.

Look at the power density of ITER (0.05 MW/m^3), of ARC (0.5 MW/m^3), and a PWR reactor vessel (20 MW/m^3).