How much does this save.....
On the pre-380 Desiro designs, quite a lot as there would be a much larger wiring harness that would need to pass throughout all of the units.
On 380s and Desiro City models, it comes as part of a software modification because doors, motor controls etc, are all controlled by a micro controller translated multi drop cable, so it would simply be a software addition into the uCs with a bit of extra communication, into the layers. Since each carriage or door will have an ordered ID (thanks to working on a multidrop cable) will mean you can select SDO using the TMS with nothing more than a software modification.
For 350s, you'd need to add a heck of a lot more wiring, or place door controls at each one and run it as a control de-select where everything in front or behind of the selected door, as works on Mk3s.
Of course on any Desiro units you can simply lock a desired door out of use, as is done on 185s doubled up at Salford Crescent, and the lockout enables the control lock out on that set of doors, so it indicates "Door not in use" and will not illuminate the controls.