How much discount for duplex with basement?

Questions to think about:
1) How much above or below grade is the basement?
2) How tall are the ceilings?
3) How much light is there?
4) Is it front or rear facing?
5) Level of noise from outside and from any building mechanicals/common spaces?
6) Does it face a trash area, or is it in close proximity one (indoor or out)?
7) Can the space down there legally be called bedroom(s)? (Most 1st and basement duplexes have the bedrooms in the basement.)
8) Is there a full bath down there?

And probably others.

Depending on the answers to the above, the amount of the discount could be anything from 0% (for English basement type spaces, esp. if rear facing and with a yard) to maybe 50% or more (for fully below grade space with no windows that can only be used as a "rec room").

It's a front facing unit. The street generates LOTS of traffic.
Separate entrance in the basement + 1/2 bathroom.
I don't have more details for now.

Actually, i do. There are windows facing a wall. Walkers can't access the windows as the spacing between the building facade and wall is roofed with metal grid.

Much like outdoor space, the value of a basement or cellar can vary dramatically. I own a front facing ground floor/cellar duplex- and I love it.I doubt I would feel the same if I lived on a heavily trafficked block.

One other factor that wasn't mentioned was whether it was a finished space or not- not all basements are finished. If it is finished- what level are the finishes.