First guess 800-1200$ with a little luck and the right buyer, who needs a 1-char for some project but at limited budget.
Value for 1.mr or 8.mr would surely be bigger. 8.mr could be associated with "Mr. Lucky".
Some explanation why I see the value low:
I really like the "MR" since its easy remembered and can be associated with "Mister" or "Mixed Reality", so this makes them very nice domains.
Still international organizations rate the human rights in Mauritania as problematic ( have a look at here for example:
https://en.m.wikipedia.org/wiki/Freedom_of_religion_in_Mauritania or here
https://en.m.wikipedia.org/wiki/Slavery_in_Mauritania ). Maybe somebody here who can share some more insight on this matters? It would be interesting to know more views.
Also in terms of GDP and HDI the values are low what might affect price. So from one side it's a beautiful domain but in the other hand there are factors that can lower the value because potential buyers might opt out to other domains.
The "0" also limits the markets where the domain might be popular though not necessarily - some people may like that.
I hope to see other opinions here too