I've become a regular at Luca Cucina Italiana, where a bowl of polenta soup goes for $3.50. They start with a beautiful tomato sauce, add the polenta, and top it with a slice of mozzarella. My only complaint is that I want a tad more tomato sauce than they normally use, but they're happy to add a bit extra. Basic, no frills spot, but I've loved everything I've had there.
Other contenders for something that good at that price?
Luca Cucina Italiana
2057 San Pablo Ave, Berkeley, CA 94702