No idea about the DR400.
I was also confused by the cost of the encoder. The excuse I was told "that's the expensive one, there is a cheaper one but it would be really tough to fit properly (which I have since discovered is total codswollop)" You may make your own assessment. The same place also charged us several hundred pounds each year for a "Radio Annual" which isn't necessary or required as part of the ARC renewal but sadly there is nowhere else to go for avionics. You live and learn.