Nachos and a taco at Green Burrito (same store offshot of Carl's Jr)
Don't know how widespread they are, but in some Carl's Jr locations here in the SF Bay Area have what's called Green Burrito--fast food Mexican in the same vein as Taco Bell and Del Taco.
What prompted me to seek it out was a craving for quick & cheap nachos (and no I didn't want to spend time buying all the ingredients & assembling it on my own) during the viewing of an NHL playoff game a few weeks ago.
The first place I thought of was Taco Bell since they're so ubiquitous. I got the most expensive version ($3.69?) and was not impressed. Basically ground beef with some tomatoes thrown in and topped with cheese sauce and the amount of food to cost ratio was low. The chips were thin & light, though.
Take two a few playoffs games later. I got the basic beef Super Nachos from the local Green Burrito. It broke the $4 barrier ($4.29) but you definitely get your money's worth--a clamshell takeout container loaded with chips, ground beef, sour cream, guacamole, tomatoes, and peppers! There's also a salsa bar where you can not only add salsa but yellow peppers and jalapenos. and for an upcharge you can get steak or chicken instead.
I did try a non-takeout version later and the portion given was smaller when I ate in. My suggestion is to order inside to go and get additional toppings from the salsa bar.
Finally, I tried their cheap taco and for a fast food taco, I was pretty impressed. The shell was double-walled unlike the usual single wall shell you see elsewhere. The beef was seasoned and had a little bit of spicyness--pretty benign but definitely noticeable.
In closing, I would not go out of my way to seek one out but if you really want fast food Mexican, this is the place to go!