Martian Air Pressure, How much variance is there? Options

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Just thinking when looking at the Victoria pics...how much does the average air pressure increase with respect to the surrounding area at the bottom of the crater? Also, where is the lowest point on Mars (which also presumably has the highest air pressure)? I can't remember if it's in Hellas or Coprates Chasma.

Rationale here is that, given MRO's astonishing capabilities, these low areas should be given priority for long-term monitoring; if transient surface water exists anywhere, it's probably way down in a crater or a canyon.

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Wow...comprehensive & well-stated, J; thank you very much for the scholarly response! I completely agree that we have a high-priority target region for long-term monitoring by MRO based on your research!!!

Now HERE'S a wild thought: What do you suppose the odds are that Mars has deep cave networks al a Carlsbad Caverns in the US? Such features might have been formed during volcanic events and/or any possible wet & warm epoch(s) a few billion years ago. This is way off the cuff, but what would the atmospheric pressure be in a cave that's maybe two miles below local ambient MSL ("S" in quotes)?

[EDIT] Yeah, I saw Robinson Crusoe On Mars many years ago (great flick, BTW)...still gotta wonder about the conditions inside deep Martian caves, if in fact there are any.