It seems to be a common misconception that lowering an object's orbit is much easier than raising it. I guess because it feels like it is just floating up there and gravity can do most of the work to pull it down if you give it a little nudge. In reality the energy required to lower the orbit is about the same as the energy required to raise it. In both cases you need to change the orbital velocity.

This is basically what they’re doing; most of the cost is for SpaceX to modify one of their existing vehicles into the USDV. You probably underestimate how much energy is needed to deorbit ISS.