Average sized adult males need at least 10 feet by 10 feet (3 meters by 3 meters), and that will feel cramped. More is better. 7x7 meters square is desirable.
If you are doing research, start with Dollamur and Tiffin to see what high-quality mats cost, and then work downward in cost/thickness/quality. Big judo throws and wrestling slams need thicker better mats (especially if you are putting them down on concrete), but almost anything is fine just for grappling.
I suggest going on Craig's List (or whatever the equivalent is, in San Diego): search for 'mat', 'wrestling mat', 'tumbling mat', and see who is getting rid of what.