I have 4.10 in my e90 M from Dinan. When I got it and it depends how close you are with your dealer but I got it for 10% off and I believe Dinan sells them at 2499 plus your original diff.
Not sure if they have a policy if your diff for some reason is damaged, but I swapped mine out pretty early in the cars life once I determined if it was necessary for the car.
One of the best modifications I have done besides the SC and MONOBALL kit.
The Dinan diff is exceptional and has held up extremely well and is exactly what this car needs if you have a manual. The only part missing is the somewhat sloppy shifter in the M. Still have not decided if a SSK is worth it, but for the 4.10 it seems like (in my opinion) the shifter just feels weak and made of plastic. I guess hard to explain. Just love my TTRS shifter and how it feels when throwing it from gear to gear. Wish I could put this in the M.
If you are considering it..do it well worth the $$ and won't regret the purchase.
The power in 1-3 is fantastic and has added good low end response. Great for track and spirited driving. Can be a bit rough at times in traffic though if a DD.
PM me if you want to talk more. If you are in New England area I would be great to meet up so you could get a feel of it.