What's the mass of the interstellar neighborhood of the Sun? What's approximatively the mass of the interstellar neighborhood of the Sun ?
(for example the neighborhood of radius 50 light-years). 

Remark: The mass of the observable matter should be completed by the mass of the "dark matter".
You don't have to guesstimate to come up with the answer.

What you do is look at the dynamics of stars with respect to the Galactic plane - in particular, the velocity dispersions of stars with known distances from the plane, combined with a reasonable assessment of where the Sun is with respect to the plane (close), yields an almost model-independent assessment of the mass density in the disk of our Galaxy in the vicinity of the Sun.

This work was done using the Hipparcos cataogue by Creze et al. (1998). They found that the local mass-density (all forms of matter) in the Galactic disk was $0.076 \pm 0.015 M_{\odot}/pc^3$.

Thus within 50 light years (15.3 pc), because the Galactic disk has an exponential scale height much bigger than this (about 100-200 pc), we can assume a constant density and derive a total mass in the solar neighborhood of $1100\pm 200\ M_{\odot}$.

This mass is almost compatible with the total mass estimated to be in stars, white dwarfs, neutron stars, brown dwarfs and the interstellar medium gas. For instance, according to Chabrier (2001), the total mass density is almost accounted for in the form of 60% main sequence stars plus white dwarfs and neutrons stars, about 5% brown dwarfs and 30% gas, and so the contribution of dark matter to the local disk density is probably very small. Dark matter is not distributed in the same way as normal matter; it did not collapse to a disk, because it is dissipationless and probably exists in a spherical halo. So this consistency between the local baryonic matter density and the total mass density is not an argument against the dark matter hypothesis. 

EDIT: I found a few estimates of the local dark matter density that use the kinematics of local stars as a constraint. The range of densities quoted is from 0.008 to 0.02 $M_{\odot}$/pc$^3$ (Bovey &amp; Tremaine 2012; Garbari et al. 2012) i.e. small compared with the contribution from stars and gas.