That rent seems very, very cheap for San Diego, unless you are way out in Alpine or something similar.
Also, not only are you getting super cheap rent, you are essentially investing in your inheritance. My situation is different in that I wanted to re-fi and mentioned to my dad my frustration with not being able to because it is no longer owner occupied, and he offered to lend me the money. I pay him a set, fair-market interest rate (with a lien and income tax filings so it is all official). He gets a guaranteed return on his money, I get an interest rate I couldn't have otherwise (because we moved overseas, not due to credit or a rate below those available at the time). In addition to that, my interest is going to him instead of to a bank. I will someday inherit 50% of the estate, so in effect I am getting 50% of that interest back.
In your case, assuming your mom dies with a positive net worth, you'll see that rent again someday. And you certainly can't buy anything remotely comparable for that payment. If you factor in taxes and everything else, I'm not sure you can find too many 1-2 bedroom townhouses for that payment, unless you are willing to live way far out and likely deal with very long drives.