I am a Quaife dealer. The qdf5v is 1412.00. It is fairly expensive compared to the other units. Of the top of my head I thought the cut of was at 2.7- gears. But it looks like 2.65. It fits the older 210 differentials from the w107 cars up to the w129-w140 case. Also used in the w124 w210 etc. At around 2000 Mercedes switched carriers to a wider unit. This uses the qdf7v. Still a 210mm carrier. They are roughly the same price. The qdf9v in the 215mm case is 1000.00.
Honestly at the additional price for the Quaife I would still buy it over and over instead of the factory 210mm lsd that was in the old cars. It lasts about a day or two if you drive it hard and has pretty low lockup. Then you have to buy another or rebuild it. The Quaife unit has a lifetime warranty. Has no clutches to wear out, is seamless in it operation and has torque bias to what you need to keep both wheels pushing hard. Absolutely the best upgrade I have ever made. I recommend it to anyone making more than 50hp.
As far as cost goes. I know it is expensive. But we are in a limited market. Especially in the older units. It is all volume sales. A couple of years ago the qdf9v was down about 200.00 from where it is now due to srt8 cars using the same diff. If we get volume up on the the qdf5v then costs go down. Right now I order them one at a time from the UK. If I remember right 10 or more starts making significant savings. Like 15%.