Definitely expensive, but it is likely still much less expensive than putting all of the satellites in to the higher orbit. It takes a lot more fuel to get the satellites in to the higher orbit, as well as still having enough fuel to get it back down to de-orbit the satellite.

Plus, in the ~1100 km orbit you are getting in to the lower Van Allen belt where the radiation will degrade electronics much more quickly. This can be mitigated with radiation-resistant parts and more shielding, but it adds a lot more cost.

There are new technologies like air breathing ion engines and electrodynamic tethers, which have the potential to sustain low orbit satellites for years.

Isn't the lower orbit even better for them in terms of latency though? So long as the orbit doesn't decay faster than the planned 5 year life span of the sats, it's actually a win.