There's two banh mi places side by side. This place and "Che Cali" over to the left. I tried both in a head to head and Nguyen Huong came out on top.
Signage is all in Vietnamese and you'll find yourself a bit out of your comfort zone upon walking in. Do the white guy thing and start flapping your arms around, singing something from Glibert & Sullivan, and eventually you'll be gently guided to the sandwich counter on the North side of the store.
Best I can figure there's a small and large sandwich. You can have it spicy or mild. I tried the mild pork large. $2.25 with tax. Those into a footlong at Subway might want to order two... bringing you in at $4.50.
Yeah. $4.50. Two street dogs will run you $5-$6 in Toronto.
There's no seating, as others have noted, but you could walk south to one of the Chinese almost-malls and munch it there. Or go east on Dundas to the Villiage on the Grange and mow it in their foodcourt.