How can I find the depth of a portion of the Valles Marineris?
You can find MOLA laser altimeter data here:https://pds-geosciences.wustl.edu/missions/mgs/megdr.html.  The files are simple rectangular grids of differences between Martian "sea-level" and measure height at 1/128 degree resolution.  If you have the latitude and longitude, it is quite straightforward to find the height.

To view these files, you probably need the software found here: https://pds.jpl.nasa.gov/tools/about/pds3-tools/nasa-view.shtml 

Looks like the software might be a little out-of-date, but you can give it a try.