I believe Banh Mi is a generic term for bread. You can get Vietnamese sandwiches with any number of fillings, such as Nem Nuong (grilled pork), Bo Thit Nuong (grilled beef), and so on.
There are lots of these places in Little Saigon and the West San Gabriel Valley to a lesser extent. Given how cheap they are, you can probably afford to order just about every sandwich on the menu and still have some change leftover.
In which part of LA do you live?
Banh Mi My Tho on Valley in Alhambra has really goods ones, but I'm partial to the "charbroiled" pork filling. The place is tiny and there's always a line when we go, but well worth the wait. The prices are in the range that DU mentioned, and as he also mentioned, there's a lot more going on with banh mi than just pork rolls (I'm guessing you're referring to what is called pork loaf). Work your way through the various menus at the many shops and you'll probably come up with some more favorites.
The best aren't in LA proper but out in Alhambra/San Gabriel and down in Westminster/Garden Grove. Do a search for banh mi for specific reccos. Also, there are far more filling possibilities for great banh mi than just pork. :-)
You'll find the going price is $2.25-$2.75 per. The Banh Mi & Che Cali chain offer a buy-2-get-1-free offer so the effective price is $1.67 each.