There are probably tons more authentic Mexican places scattered about Portland and the greater area, but the safe bet for an inexpensive quality burrito - Cha Cha Cha. Besides having a fun name, the locations are many and cool. Two downtown (one near PSU, the other in the Pearl), one on SE Hawthorne and the last in Sellwood. Almost all locales are houses cum restaurants giving Cha that homey feel. Mostly, I've tried the burrito selection, finding the pinnacle of their Burrito Technology to lay in the aptly named Cha Burrito. The tacos are worth trying too; they are flat and round in the traditional style. You might want to skip the fish taco, however, as I received a hunk of breaded fish that would only have been appropriate if I was eating fish and chips. Their salsa verde is killer and the pico de gaillo is not bad either. Finally, cool your mouth down with an excellent horchata just before you add more hot sauce for punishment's sake.